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Plant Physiol. Aug 2005; 138(4): 2145–2154.
PMCID: PMC1183402

Expression of Arabidopsis MIRNA Genes1,[w]

Abstract

MicroRNAs (miRNAs) are approximately 21-nucleotide noncoding RNAs that regulate target transcripts in plants and animals. In addition to miRNAs, plants contain several classes of endogenous small interfering RNAs (siRNAs) involved in target gene regulation and epigenetic silencing. Small RNA libraries were constructed from wild-type Arabidopsis (Arabidopsis thaliana) and mutant plants (rdr2 and dcl3) that were genetically enriched for miRNAs, and a computational procedure was developed to identify candidate miRNAs. Thirty-eight distinct miRNAs corresponding to 22 families were represented in the libraries. Using a 5′ rapid amplification of cDNA ends procedure, the transcription start sites for 63 miRNA primary transcripts from 52 MIRNA loci (99 loci tested) were mapped, revealing features consistent with an RNA polymerase II mechanism of transcription. Ten loci (19%) yielded transcripts from multiple start sites. A canonical TATA box motif was identified upstream of the major start site at 45 (86%) of the mapped MIRNA loci. The 5′-mapping data were combined with miRNA cloning and 3′-PCR data to definitively validate expression of at least 73 MIRNA genes. These data provide a molecular basis to explore regulatory mechanisms of miRNA expression in plants.

MicroRNAs (miRNAs) are approximately 21-nt noncoding RNAs that posttranscriptionally regulate expression of target genes in plants and animals (Bartel, 2004). Mature miRNAs are generated through multiple processing steps from primary transcripts (pri-miRNA) that contain imperfect foldback structures. In animals, MIRNA genes are transcribed by RNA polymerase II (pol II; Bracht et al., 2004; Cai et al., 2004; Lee et al., 2004), yielding a pri-miRNA that is processed initially by the nuclear RNaseIII-like enzyme Drosha (Lee et al., 2003). The resulting pre-miRNA transcripts are transported to the cytoplasm and processed by Dicer to yield mature miRNAs (Lee et al., 2002). Less is known about the miRNA biogenesis pathway in plants, although most or all miRNAs require Dicer-like1 (DCL1; Park et al., 2002; Reinhart et al., 2002). The lack of a Drosha ortholog in plants and the finding that DCL1 functions at multiple steps during biogenesis of miR163 suggest that plant miRNA biogenesis may differ somewhat from animals (Kurihara and Watanabe, 2004). miRNAs in both animals and plants incorporate into an effector complex known as the RNA-induced silencing complex and guide either translation-associated repression or cleavage of target mRNAs (Bartel, 2004).

Computational and molecular cloning strategies revealed nearly 100 potential MIRNA genes in the Arabidopsis (Arabidopsis thaliana) genome (Llave et al., 2002a; Mette et al., 2002; Park et al., 2002; Reinhart et al., 2002; Palatnik et al., 2003; Jones-Rhoades and Bartel, 2004; Sunkar and Zhu, 2004; Wang et al., 2004). These miRNAs target mRNAs encoding proteins that include a variety of transcription factors involved in development, miRNA/small interfering RNA (siRNA) metabolic or effector components (DCL1, Argonaute1 [AGO1], and AGO2), components of the SCF complex involved in ubiquitin-mediated protein degradation, several other classes of metabolic and stress-related factors, as well as trans-acting siRNA (ta-siRNA) primary transcripts (Llave et al., 2002b; Park et al., 2002; Rhoades et al., 2002; Aukerman and Sakai, 2003; Emery et al., 2003; Kasschau et al., 2003; Palatnik et al., 2003; Tang et al., 2003; Xie et al., 2003; Achard et al., 2004; Allen et al., 2004, 2005; Chen, 2004; Jones-Rhoades and Bartel, 2004; Laufs et al., 2004; Mallory et al., 2004a; Sunkar and Zhu, 2004; Vaucheret et al., 2004; Vazquez et al., 2004). Based on tissue distribution and limited in situ expression data, most plant miRNAs are likely regulated during development (Chen, 2004; Juarez et al., 2004; Kidner and Martienssen, 2004; Parizotto et al., 2004). Overexpression or knockout of MIRNA genes, or expression of MIRNA genes outside of their normal domains, can lead to severe developmental defects (Aukerman and Sakai, 2003; Emery et al., 2003; Palatnik et al., 2003; Achard et al., 2004; Chen, 2004; Juarez et al., 2004; Kidner and Martienssen, 2004; Laufs et al., 2004; Mallory et al., 2004a, 2004b; McHale and Koning, 2004; Zhong and Ye, 2004). Understanding the mechanisms governing MIRNA gene expression patterns is necessary, therefore, to understand miRNA-mediated regulatory pathways and networks.

In this study, new Arabidopsis miRNAs were identified or validated by a computationally assisted cloning approach and the use of miRNA-enriched mutants. Features associated with transcription initiation of over one-half of all known Arabidopsis MIRNA genes were analyzed, revealing start sites, core promoter, and other properties that were consistent with a pol II mechanism of gene expression.

RESULTS AND DISCUSSION

Identification and Validation of Arabidopsis miRNAs

Several small RNA libraries were constructed from wild-type (Columbia [Col]-0) Arabidopsis seedling and inflorescence tissues, and from aerial tissues of jaw-D plants that overexpress miR-JAW (miR319; Palatnik et al., 2003). Among the 2,357 sequences analyzed collectively from these libraries, only 32.7% corresponded to known or subsequently validated miRNA families. Most of the remaining small RNAs corresponded to diverse sets of endogenous small RNAs arising from sequences such as transposons, retroelements, simple sequence repeats, inverted duplications, rDNA genes, and other genic and intergenic sequences (Llave et al., 2002a; Xie et al., 2004). To genetically enrich for miRNAs, small RNA libraries were constructed from embryo, seedling, and inflorescence tissues of rdr2-1 mutant plants and from seedlings of dcl3-1 mutant plants. These plants contain relatively low levels of approximately 24-nt siRNAs from repeated sequences but maintain normal levels of miRNAs (Xie et al., 2004). Among 3,164 sequences analyzed from the rdr2-1 and dcl3-1 libraries, 70.5% corresponded to previously characterized miRNAs, representing a 2.2-fold overall enrichment relative to the wild-type libraries. Endogenous siRNAs from known repeat families (identified from RepBase) were reduced 43.9-fold in the mutant libraries. The majority of the remaining small RNAs corresponded to sequences from two RDR2-independent small RNA-generating loci, or from rDNA genes. Unique miRNA and endogenous siRNA sequences from all libraries are available in the Arabidopsis Small RNA Project (ASRP) database (http://asrp.cgrb.oregonstate.edu/; Gustafson et al., 2005).

To identify new miRNAs in the cloned libraries, small RNA sequences were subjected to a series of six computational filters (Fig. 1A). The filters were designed using consensus properties of a founder set of published, validated Arabidopsis miRNAs with codes within the range of miR156 to miR399 (excluding miR390 and miR391; Llave et al., 2002a; Mette et al., 2002; Park et al., 2002; Reinhart et al., 2002; Palatnik et al., 2003; Jones-Rhoades and Bartel, 2004; Sunkar and Zhu, 2004), and using consensus properties of miRNAs from plants and animals (Ambros et al., 2003; Griffiths-Jones et al., 2003). Among the 48 unique miRNA sequences from 92 loci (22 validated miRNA families) in the founder set, 34 miRNA sequences from 71 loci (19 families) were in the cloned database. The initial filters eliminated small RNA sequences deriving from structural RNA genes, other annotated genes, and repetitive loci identified by RepeatMasker (Fig. 1A). Sequences originating from loci that yielded multidirectional clusters of small RNAs, which is a hallmark of many siRNA-generating loci, were eliminated. Small RNAs that were not 20 to 22 nt in length, based on the cloned sequence, were removed. Small RNAs originating from loci that lacked the potential to form a miRNA precursor-like foldback structure, consisting of a stem in which 16 or more positions within the putative miRNA:miRNA* duplex region were paired, were excluded. To test sensitivity, the complete founder set of miRNAs was processed through these filters. All but three passed, corresponding to a false negative rate of 0.032. miR163 failed due to length (24 nt), and two MIR166 loci (c and d) failed because six or more mispaired bases were located within the miRNA/miRNA* region in the predicted foldback. From the cloned dataset, a total of 103 small RNAs passed each filter (Fig. 1A). These did not correspond to 103 unique loci, however, as many miRNA-generating loci yield multiple processed forms that were offset by one or a few nucleotides. The final filter eliminated all sequences corresponding to founder miRNAs, which yielded 18 small RNAs (13 loci) as candidate new miRNAs (Fig. 1A; Supplemental Table I). This set included miR390, miR391, miR403, and miR447 (Fig. 1B). miR403 and miR390 were also identified in an independent small RNA library (Sunkar and Zhu, 2004). Six of the 18 small RNAs corresponded to a cluster of processing variants from the two MIR390 loci.

Figure 1.
Cloning and identification of Arabidopsis miRNAs. A, Flowchart for identification miRNAs in cloned small RNA libraries. The number of small RNAs passing each filter is shown in parentheses. B, Predicted precursor foldback structure of miRNAs (red) validated ...

Given the sensitivity of the computational filters using the founder set, a second set of published Arabidopsis sequences with miRNA designations were analyzed. This set, which has not been subjected to extensive experimental validation, includes all sequences with codes between miR400 and miR420 (Sunkar and Zhu, 2004; Wang et al., 2004), except miR403. In contrast to the founder set, most of these small RNAs failed at one or more computational steps. Six small RNAs (miR401, 405a–d, 407, 416) were identified as transposon derived, and 10 (miR401, 404, 406, 408, 413, 414, 417–420) failed the foldback prediction criteria. Given the high computational failure rate (0.84), which was 26-fold higher than the false negative rate of the founder set, it is likely that many of these are endogenous siRNAs and not bona fide miRNAs.

Candidate miRNAs from each of the 13 loci identified in the computational analysis (Fig. 1A; Supplemental Table I) were subjected to validation-blot assays using a series of Arabidopsis miRNA-defective (dcl1, hyl1, hen1, and hst) and siRNA-defective (dcl2, dcl3, rdr1, rdr2, rdr6, and sgs3) mutants (Reinhart et al., 2002; Kasschau et al., 2003; Jones-Rhoades and Bartel, 2004; Peragine et al., 2004; Vazquez et al., 2004; Xie et al., 2004; Allen et al., 2005). The previously validated miR173, AtSN1-derived siRNAs and ta-siRNA1511 were analyzed in parallel as controls. miR173, miR403, miR390, miR391, and miR447 each accumulated to relatively low levels in the dcl1-7, hen1-5, and hyl1-2 mutants, but accumulated to normal or near-normal levels in the dcl2-1, dcl3-1, rdr1-1, rdr2-1, rdr6-15, sgs3-11, and zip1-1 mutants (Fig. 1C). The hst-15 mutant had a moderate effect on accumulation of all miRNAs (Fig. 1C), as shown previously (Park et al., 2005). Based on structural and biogenesis criteria, as well as target validation data (Allen et al., 2005), we conclude that miR390, miR391, miR403, and miR447 are bona fide miRNAs. Small RNAs from the remaining eight loci (Supplemental Table I) were not detected in blot assays and were not characterized further.

The targets for miR390, miR403, and miR447 were recently predicted and validated (Allen et al., 2005). Genes encoding AGO2 (At1g31280) and a 2-phosphoglycerate kinase (2PGK, At5g60760) were validated as targets of miR403 and miR447, respectively (Table I; Allen et al., 2005). Interestingly, miR390 was shown to target primary transcripts from a ta-siRNA-generating locus (TAS3; Table I; Allen et al., 2005), setting the phase for subsequent processing of pre-ta-siRNAs through the RDR6/SGS3-dependent pathway (Allen et al., 2005).

Table I.
Arabidopsis miRNA families

miR390 and miR391 are related miRNAs that differ by 5 nt, whereas miR403 and miR447 are distinct from all other known miRNAs. If miR390 and miR391 are assigned to the same family, then Arabidopsis contains at least 25 experimentally validated families of miRNAs encoded by up to 99 genes (Table I). Among these families, 19 are conserved between dicots and monocots. One family (miR403) is conserved among families within dicots, and five families (miR158, miR161, miR163, miR173, and miR447) have been identified only in Arabidopsis.

Arabidopsis miRNA Primary Transcripts

To determine if a reference set of three Arabidopsis MIRNA gene transcripts contains 5′-cap structures typical of RNA pol II transcripts, a series of RNA ligase-mediated (RLM)-5′RACE reactions were done using poly(A+)-selected RNA that was pretreated with either calf intestine phosphatase plus tobacco acid pyrophosphatase (CIP + TAP) or buffer alone. Only transcripts containing a 5′ cap should ligate to adapters, and subsequently amplify by PCR, following CIP + TAP treatment. Transcripts lacking a cap should ligate and amplify only from the sample treated with buffer alone. As controls, capped Scarecrow-like6-IV (SCL6-IV, At4g00150) mRNA and miR171-guided 3′-cleavage product from SCL6-IV (containing a 5′ monophosphate) were analyzed using gene-specific primer sets (Fig. 2A; Llave et al., 2002b). CIP + TAP-dependent 5′RACE products of the predicted size, approximately 400 and approximately 1,110 bp, were detected using 5′-proximal and cleavage site-proximal primer sets, respectively (Fig. 2B, lanes 2 and 4). CIP + TAP-independent 5′RACE product was detected only using the cleavage site-proximal primer set (Fig. 2B, lanes 1 and 3). Using locus-specific primer sets for MIR163, MIR397b, and MIR398c, CIP + TAP-dependent products, but not CIP + TAP-independent products, were detected (Fig. 2B, lanes 5–10), indicating that the 5′ end of each miRNA transcript was capped. For 52 out of the 99 Arabidopsis MIRNA loci tested, 5′RACE products from poly(A+)-selected and 5′-capped RNA were detected (see below and Supplemental Table II). Recently, a diverse set of putative miRNA transcripts from more than 30 plant species were identified from expressed sequence tag (EST) databases, suggesting that plant miRNA precursor transcripts are likely polyadenylated (Jones-Rhoades and Bartel, 2004). These data, combined with previous characterization of MIR172b and MIR163 transcripts, indicate that plant MIRNA genes are transcribed by an RNA pol II mechanism. These data are also consistent with recent analyses of MIRNA gene transcripts from animal systems (Bracht et al., 2004; Cai et al., 2004; Lee et al., 2004).

Figure 2.
RLM-5′RACE on MIRNA transcripts. A, Schematic representation of a generic MIRNA transcript, and SCL6-IV mRNA (5′ cap-containing control) and miR171-guided cleavage product from SCL6-IV mRNA (noncapped control). The relative positions of ...

Identification of Core Promoter Elements for Arabidopsis MIRNA Genes

For 52 of the 99 MIRNA genes tested, 5′RACE products were detected using locus-specific primers. In most cases (41 loci), positive 5′RACE products were detected as a uniform-sized fragment. At the remaining 10 loci, however, multiple 5′RACE products were detected. Nine of these (MIR156a, MIR156c, MIR160a, MIR162a, MIR164a, MIR172a, MIR172c, MIR394a, and MIR447b) gave rise to 5′RACE products of two distinct sizes. The other two, MIR172b and MIR172e, gave rise to three distinct 5′RACE products. Each PCR product from the 52 positive loci was cloned and sequenced, and transcription start sites were inferred based on the most abundant 5′ position represented among six or more clones randomly selected for sequencing. In cases where two 5′ positions were represented equally from one PCR size class, the extreme 5′ sequence was assigned as the start site. At 10 of the 11 loci (with the exception of MIR172b) for which multiple 5′RACE products were detected, alternative transcription start sites were identified. In the case of MIR172b, the three 5′RACE products corresponded to alternatively spliced transcripts that initiated at the same start site (Supplemental Fig. 1). Thus, 5′ ends representing 63 transcripts from 52 MIRNA loci were identified (Fig. 3). The vast majority (86%) of transcripts initiated with an adenosine, of which 93% were preceded by a pyrimidine (Fig. 3A). These characteristics are consistent with transcription by RNA pol II (Lorkovic et al., 2000; Shahmuradov et al., 2003).

Figure 3.
Transcription initiation sites of Arabidopsis MIRNA primary transcripts, and core promoter elements. A, Base composition at positions flanking MIRNA transcription initiation sites (n = 63). B, Genomic sequence of 60 nt flanking each of 63 MIRNA ...

Several characteristics of MIR163 and MIR172b primary transcripts were reported in two previous studies (Aukerman and Sakai, 2003; Kurihara and Watanabe, 2004). The MIR163 transcription start site mapped here is identical to the site identified previously (Kurihara and Watanabe, 2004). Splicing variants for both MIR163 and MIR172b were reported (Aukerman and Sakai, 2003; Kurihara and Watanabe, 2004). We identified two introns (117 nt and 199 nt) separated by 41 nt upstream of the predicted foldback structure in the MIR172b transcript (Supplemental Fig. 1). Only one intron was reported in this region by Aukerman and Sakai (2003). The MIR163 intron was not detected in this study, as 5′RACE primers corresponded to a sequence upstream of the intron. An intron was also identified in one of the two MIR156a transcripts (Supplemental Fig. 1). In each case, the intron began with 5′-GU and ended with AG-3′, as is typical of group III introns that are commonly found in pre-mRNAs of higher plants (Lorkovic et al., 2000).

To identify conserved motifs flanking the initiation sites at each mapped locus, a 60-bp genomic segment (−50 to +10 relative to the start site) was computationally analyzed using BioProspector (Liu et al., 2004). An 8-nt TATA box-like sequence was identified as a conserved motif in this region (Table II). This motif was detected upstream from 52 (83%) of the mapped transcription start sites (Fig. 3B). To determine if the high frequency occurrence of the TATA box-like sequence was uniquely associated with this specific region, we examined the distribution of the TATA box-like sequence in an extended upstream region (−200 to +50) using MotifMatcher. The TATA box-like sequence centered at consensus position −29 from the start site (Fig. 3C), which is entirely consistent with TATA box motifs located in protein-coding genes (Patikoglou et al., 1999; Shahmuradov et al., 2003). We conclude, therefore, that most or all of these motifs correspond to authentic TATA box sequences within core promoters of MIRNA genes.

Table II.
Position-weight matrix for conserved TATA box-like motif

Expression of Arabidopsis MIRNA Genes

Despite repeated attempts with multiple primer sets, 5′RACE products were detected from only 53% of MIRNA genes tested (Fig. 4). This may have been due to low levels of expression of some MIRNA genes in tissues analyzed or lack of expression of some loci predicted to be MIRNA genes. It is also possible that some primer sets were inadvertently designed within intron sequences or that some miRNA sequences derive from non-5′ positions within polycistronic primary transcripts. To develop a more comprehensive account of expression of Arabidopsis MIRNA genes, informatic and experimental approaches were taken. In the informatic strategy, the ASRP database was scanned for miRNA or miRNA* sequences corresponding to loci with negative 5′RACE results (Gustafson et al., 2005). Unique miRNA or miRNA* sequences from MIR156d, MIR158a, MIR164c, MIR167d, MIR168a, MIR169b, MIR169i, MIR169m, MIR173, MIR390b, MIR391, MIR397a, and MIR403 loci were each represented in the database (Fig. 4). For MIR168a, additional evidence confirming expression came from a locus-specific EST clone (GenBank accession no. H77158). In addition, unique miRNA sequences specific to MIR319c, MIR390a, MIR398a, and MIR399f were each represented in an independent Arabidopsis small RNA library (Fig. 4; Sunkar and Zhu, 2004).

Figure 4.
Locus-specific expression of 99 predicted MIRNA genes encoding validated miRNAs in Arabidopsis. Expression of a specific locus was considered definitive (dark green shading) if a primary transcript was detected by 5′RACE or 3′RACE, or ...

For the remaining predicted MIRNA genes, locus-specific primers were designed to amplify sequences immediately downstream of the precursor foldback sequence using a 3′RACE procedure. Three 5′RACE-positive MIRNA loci (MIR162a, MIR162b, and MIR403) were also included in the 3′RACE analysis as controls. Positive results were obtained for MIR157a, MIR157b, MIR166e, MIR166f, and MIR398b as well as for the three control loci (Fig. 4). For the miR393 family and a subset of members in the miR169 family (MIR169dg), neither 5′RACE nor 3′RACE yielded positive results (Fig. 4), although expression from at least one locus was inferred based on detection of a sequence in at least one small RNA library (Jones-Rhoades and Bartel, 2004; Sunkar and Zhu, 2004; Gustafson et al., 2005). Collectively, unambiguous data support the expression of at least 73 of the 99 Arabidopsis MIRNA loci (Fig. 4).

Each of the genes encoding miRNAs that are conserved between monocots and dicots are members of multigene families. For nearly all of these families, multiple genes are expressed and presumed functional. In some cases, the family variants encode miRNAs with diverged sequences. We propose two forces are driving the evolution of these multigene families. First, expansion of MIRNA gene families facilitates regulatory diversification through acquisition or derivation of distinct control elements (Hurles, 2004). Although the extent to which MIRNA gene duplication leads to novel spatial or temporal control of family members remains to be determined, genetic data from MIR164 loci indicate divergence of regulatory specificity between closely related family members (Baker et al., 2005). And, second, family expansion provides the genes to generate novel miRNAs with unique target specificity, as proposed for the miR159/319 family and MYB/TCP gene targets (Palatnik et al., 2003). The finding that most members of conserved, multigene MIRNA families are expressed in Arabidopsis supports the idea that regulatory or functional diversification has occurred.

MATERIALS AND METHODS

Upon request, all novel materials described in this publication will be made available in a timely manner for noncommercial research purposes, subject to the requisite permission from any third-party owners of all or parts of the material. Obtaining any permission will be the responsibility of the requestor.

Cloning of Arabidopsis Small RNAs and miRNA Prediction

Extraction of low molecular weight RNA and library construction were done as described (Lau et al., 2001; Llave et al., 2002a). RNA was extracted from 3-d postgermination seedlings, embryos from developing siliques, aerial tissues including rosette leaves and apical meristems, or stage 1 to 12 enriched inflorescence from wild type Col-0, and jaw-D, rdr2-1, and dcl3-1 mutants as described previously (Palatnik et al., 2003; Xie et al., 2004). Seedling libraries were constructed for Col-0, rdr2-1, and dcl3-1, embryo libraries for rdr2-1, aerial libraries for jaw-D, and inflorescence libraries for Col-0 and rdr2-1. The procedure for small RNA cloning and sequencing was described previously (Llave et al., 2002a). A total of 2,357 sequences were determined from wild-type or jaw-D libraries, and 3,164 sequences were determined from rdr2-1 and dcl3-1 libraries. miRNAs were predicted from the cloned database of sequences using a set of six computational filters. First, structural RNAs were identified by BLAST and eliminated. Second, small RNAs from repeated sequences identified using RepeatMasker (Jurka, 2000), or from predicted protein-coding genes and pseudogenes only, were removed. Third, a small RNA cluster filter was applied to remove small RNAs within 500 nt of another small RNA in the opposite orientation. The fourth filter removed any small RNAs outside the typical size range for miRNA (20–22 nt). Fifth, sequences from within a context that failed to conform to a set of consensus characteristics of miRNA foldback structures were eliminated. The consensus criteria were (1) minimum of 16 paired bases within the miRNA:miRNA* duplex, (2) maximum predicted foldback size of 350 nt, (3) a requirement for the miRNA:miRNA* duplex to be predicted within a single foldback stem, and (4) three or fewer contiguous nonpaired bases. RNAFold in the Vienna RNA Package was used to predict potential duplexes containing the small RNA (Hofacker, 2003). The remaining small RNAs were compared to sequences of validated miRNAs by FASTA to identify previously characterized and unique candidate miRNAs.

Arabidopsis Mutants

Mutant lines for dcl1-7, dcl2-1, dcl3-1, rdr1-1, rdr2-1, hen1-5, hyl1-2, rdr6-15, sgs3-11, and zip1-1 were described previously (Park et al., 2002; Allen et al., 2004; Peragine et al., 2004; Vazquez et al., 2004; Xie et al., 2004). The hst-15 mutant was derived from a SALK_079290 T-DNA insertion line, which contains an insertion at position 1,584 from the start codon (Alonso et al., 2003).

Small RNA-Blot Analysis

Low molecular weight RNA (5 μg) from Arabidopsis (Arabidopsis thaliana) inflorescence tissue was used for miRNA- and endogenous siRNA-blot analysis as previously described (Allen et al., 2004). Probe for AtSN1-siRNA blot was described previously (Zilberman et al., 2003). DNA oligonucleotide probes specific for miR390 (5′-GGCGCTATCCCTCCTGAGCTT-3′) and miR391 (5′-TGGCGCTATCTCTCCTGCGAA-3′) were end-labeled with γ32P-ATP using Optikinase (New England Biolabs, Beverly, MA) according to the manufacturer's directions. Locked nucleic acid-modified oligonucleotides (Exiqon, Vedbaek, Denmark) specific for siR1511 (5′-AAGTATCATCATTCGCTTGGA-3′), miR447 (5′-CAACAAAACATCTCGTCCCCAA-3′), and miR403 (5′-CGAGTTTGTGCGTGAATCTAAT-3′) were used to improve sensitivity of detection (Valoczi et al., 2004). The blots were also analyzed with an oligonucleotide probe specific to snRNA U6 (5′-TCATCCTTGCGCAGGGGCCA-3′) as a loading control. Relative accumulation of small RNAs was determined using an InstantImager (Packard Bioscience, Boston, MA).

5′RACE Mapping of MIRNA Transcripts

Two Arabidopsis (Col-0) tissue preparations were used for RNA isolation: inflorescence tissues from 4-week-old plants grown under greenhouse conditions and 4-d-old seedlings grown on Murashige and Skoog media in a growth chamber. Total RNA was extracted using TRIzol reagent (Invitrogen, Carlsbad, CA), followed by column purification using an RNA/DNA midi kit (Qiagen, Valencia, CA). The extracts were subjected to two rounds of purification using oligo(dT) resin (Qiagen) for the enrichment of poly(A+) RNA. Poly(A+)-enriched RNA (125 ng/reaction) was first treated with CIP + TAP. The 5′ ends of MIRNA transcripts were then mapped by an RLM-5′RACE assay (Invitrogen). Complementary DNA (cDNA) was synthesized using random oligonucleotide hexamers as primers. A cDNA pool containing equal amounts of reaction product from each tissue was used as template in 5′RACE PCR with a primer specific to the RNA adaptor sequence and a locus-specific reverse primer. In general, a set of two gene-specific primers (RA and RB) were designed for each MIRNA locus based on sequences immediately upstream to the predicted foldback structure (Fig. 2A). In cases where the sequence context in this region did not allow designing primers with high specificity, or the size of resulting 5′RACE products was too small, an alternative set of primers (R1 and R2) were used (Fig. 2A; Supplemental Table III). The default annealing temperature in the touchdown PCR reaction was 65°C. For MIRNA loci yielding negative 5′RACE results after the second-round PCR, two additional PCR reactions with the nested primers were done with altered annealing temperatures. The PCR products from a positive 5′RACE reaction were gel purified and cloned. A minimum of six clones were sequenced for each PCR product. Sequences corresponding to transcript 5′ ends were deposited at GenBank with accession numbers listed in the supplemental materials (Supplemental Table II).

The RLM-5′RACE procedure was used to analyze the presence or absence of a cap structure on several miRNA primary transcripts. A capped mRNA [SCL6-IV] and a noncapped RNA (miR171-guided cleavage product of SCL6-IV mRNA) were used as control RNAs. Parallel RLM-5′RACE reactions were done using poly(A+)-enriched RNA that was CIP + TAP treated (selective for 5′ ends that contain a 5′ cap) or buffer treated (selective for noncapped 5′ ends).

For some MIRNA primary transcripts, 3′RACE was done using poly(A+)-enriched RNA. cDNA was synthesized using an adaptor-tagged oligo(dT) primer (Invitrogen). Gene-specific forward primers were designed for each locus tested, following the same procedure used for 5′RACE primer design (Fig. 2A). The identities of 3′RACE products were confirmed by sequencing. The sequences of all the locus-specific primers are listed in Supplemental Table III.

Computational Identification of Conserved Upstream Sequence Motifs

A 60-bp (−50 to +10) genomic sequence flanking the start site for 63 transcripts from 52 MIRNA loci was analyzed using BioProspector, a Gibbs sampling-based motif-finding program (Liu et al., 2004). Searches with a motif width of 6 to 8 nt were done. In all cases, TATA box-like sequences were identified as the only conserved motif. The presence of the conserved TATA box-like motif matrix (8-nt width) in each 60-bp genomic segment was checked using MotifMatcher, with up to three matches per segment allowed (Ao et al., 2004). The algorithm gives a score for placement of each TATA box-like sequence detected (Fig. 2B). These are log-odds-based scores calculated as ln[P(observed|PWM)/P(observed|background model)], where the numerator is the probability of the observed sequence according to the position weight matrix (PWM) representing the motif and the denominator is the probability of the sequence according to a simple Markov chain constructed by examining frequencies of nucleotide occurrences throughout a background sequence set (Ao et al., 2004). A second search by MotifMatcher was done using an extended upstream region (−200 to +50) to analyze the distribution of the putative TATA motif, with the 8-nt motif matrix generated by BioProspector as a sample motif. Up to three matches to the TATA box-like motif were allowed.

Sequence data from this article have been deposited with the EMBL/GenBank data libraries under accession numbers DQ063602 to DQ063665.

Supplementary Material

Supplemental Data:

Acknowledgments

We thank Scott Poethig for the dcl1-7 allele in Col-0 background. We also thank Adam Gustafson, Daniel Smith, and Christopher Sullivan for invaluable assistance and advice with computational resources; April Wilken for assistance in PCR and cloning; and Mark Dasenko for sequencing.

Notes

1This work was supported by the National Science Foundation (grant no. MCB–0209836), the National Institutes of Health (grant no. AI43288), and the U.S. Department of Agriculture (grant no. 2005–35319–15280).

[w]The online version of this article contains Web-only data.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.062943.

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