A visual screen was performed on a sub-library of the yeast deletion collection (Open Biosystems). A) Of the 46 deletion strains tested, approximately half of the mutants affected the localization of the GAL gene when transcription was active. Interestingly, only three mutants specifically affected the post-transcriptional tethering of the GAL-reporter gene to the nuclear periphery. B) Mutations in two nuclear exosome components/cofactors, Lrp1p and Rrp6p, are required for the post-transcriptional tethering of the GAL-reporter plasmid to the nuclear periphery. Localization of the LacO-GAL-reporter plasmid was examined in wild-type, lrp1Δ, and rrp6Δ cells grown in glucose (GLU; gray), galactose (GAL; black) or 15′ after transcription was repressed by the addition of glucose (SHUTOFF; striped). The percentage of plasmids that localize to the nuclear periphery was determined (see Materials and Methods). When transcription is active (galactose media), the localization of the GAL reporter plasmid in rrp6Δ and lrp1Δ cells is indistinguishable from that of wild-type (compare black bars). In contrast, when transcription is repressed, the GAL reporter plasmid is no longer retained at the nuclear periphery in rrp6Δ and lrp1Δ cells (compared striped bars).

Message-specific fluorescence in-situ hybridization (FISH) was performed to examine whether GAL-GFP-GALpA mRNAs are retained adjacent to the site of transcription when the nuclear exosome is inactivated. A) Wild-type, rrp6Δ, and lrp1Δ cells were examined after growth in glucose, galactose or 15′ after transcriptional repression. GFP message-specific FISH shows that dots accumulate in all three strains when transcription is activated (GAL). 15′ after transcription is shutoff, the dots persist in wild-type cells, but are no longer detectable in both rrp6Δ and lrp1Δ cells (SHUTOFF). B) To rule out the possibility that the dot disappearance was an indirect affect, we examined the kinetics of dot persistence in a temperature sensitive nuclear exosome mutant, rrp4-1. rrp4-1 cells were grown in galactose at 25°C until they reached mid-log phase and shifted to the non-permissive temperature (37°C). At the same time, transcription was repressed by the addition of glucose. FISH was performed on cells fixed at various times after transcription shutoff and exosome inactivation, and the percentage of cells containing dots was determined. A rapid loss of dots upon exosome inactivation and transcription shutoff suggests that the effect is direct.

Total RNA isolated from cells after growth in glucose, galactose or at various times after transcriptional repression was analyzed using 3′-RACE. A) A 3′-RACE was performed on RNA from rrp6Δ and wild-type cells. As expected from previous studies, we observe two different GAL1 transcripts resulting from polyadenylation sites that are approximately 60bp and 170bp downstream of the stop codon. In wild-type cells both the short and long forms of the RNA gradually disappear after transcription is repressed as would be expected for normal RNA turnover. However, in rrp6Δ cells, while the larger form gradually turns over, the amount of the smaller species actually increases 10-15 minutes after transcription shutoff. B) To verify that the increase in the smaller species is not due to a technical artifact during the PCR step of the 3′-RACE (the two species share the 5′ primer), we eliminated the larger RNA from the assay using oligo-directed RNAse H cleavage (see Materials and methods). We observe the same increase in the short form of GAL1 mRNA as seen in part A suggesting that this effect is not due to a technical artifact. C) The amount of the short GAL1 transcript that persists in rrp6Δ and wild-type cells after transcriptional shutoff was quantitated. In rrp6Δ cells, there is an approximate 5-fold increase in the amount of the short GAL1 transcript approximately 15′ after transcription is shutoff by the addition of glucose. In contrast, the short GAL1 transcript decreases by approximately 2-fold over the same time period. D) Since the 3′-RACE assay only recognizes polyadenylated RNA, we hypothesized that the increase in the short form of the GAL1 mRNA could be due to a novel polyadenylation event by the poly-A polymerase, Pap1p. We performed a 3′-RACE experiment on rrp6Δ cells as well as rrp6Δ cells containing a conditional allele of the poly-A polymerase, pap1-1. These cells were grown at the permissive temperature (25°C) in galactose then shifted to the non-permissive temperature at the same time that glucose was added to repress transcription. In rrp6Δ cells, there is a marked increase in the level of the smaller GAL1 RNA species after 15′ of transcriptional shutoff. When poly-A polymerase is inactivated, we no longer observe this increase in the smaller GAL1 mRNA indicating that Pap1p is required for the novel post-transcriptional polyadenylation event.

A) Schematic diagram of the two-plasmid system used to monitor GAL gene localization. pVS2 contains 256 LacO repeats, the GAL-GFP-GALpA reporter gene, LEU2 selective marker and a centromere. pKA67 contains Nup49-GFP, LacI-GFP, a URA3 selectable marker and a centromere (see Materials and Methods for details). B) Cells containing either a LacO plasmid expressing GAL-GFP-GALpA (pVS2) or a control plasmid lacking the GAL reporter gene (pVS1) were grown in either glucose (GLU) or galactose (GAL). The intra-nuclear position of the plasmid was monitored using de-convolution microscopy and determined to be either internal (gray) or peripheral (adjacent to or overlapping the nuclear periphery; black). When transcription is activated (GAL), the GAL reporter-containing plasmid (pVS2) becomes associated with the nuclear periphery. C) Cells containing the GAL-reporter containing lacO plasmid (pVS2) were grown in galactose and then glucose was added to repress transcription. Cells were fixed for microscopy at various time points after transcription shutoff and the localization of the plasmid was determined to be either internal (gray) or peripheral (black). As previously observed for a chromosomal GAL reporter gene, the GAL-reporter containing plasmid persists at the nuclear periphery for at least 30′after transcription shutoff.

To determine whether the GAL1 mRNA is important for post-transcriptional tethering, we compared the localization the GAL1 gene (black) with a GAL1 gene lacking the ORF (GAL1 ORF less; gray; see Materials and methods.) Cells were isolated after growth in glucose (GLU; transcription off), galactose (GAL; transcription activated) or 15′ after transcriptional repression (SHUTOFF) and the intra-nuclear location of the GAL loci were determined. As shown previously for GAL2 (Dieppois et al., 2006), the removal of the GAL1 ORF does not affect the ability of this gene to localize to the nuclear periphery when transcription is active (GAL). However, when transcription is repressed (SHUTOFF), the GAL1 ORF is no longer associated with the nuclear periphery strongly suggesting that mRNA is important for the post-transcriptional tethering of the gene to the nuclear periphery.

When the nuclear exosome is inactivated, the GAL-GFP-GALpA gene exhibits neither post-transcriptional tethering to the nuclear periphery nor post-transcriptional dot persistence. To test whether cleavage and/or polyadenylation is required for these effects, we repeated the experiments using a reporter, GAL-GFP-RZ that cannot be polyadenylated. This reporter is identical to GAL-GFP-GALpA except that the GAL1 3′UTR is replaced by a self-cleaving hammerhead ribozyme. A) The localization of GAL-GFP-GALpA and GAL-GFP-RZ in both wild-type and rrp6Δ cells was examined over time after transcription repression. As shown previously, both GAL-GFP-GALpA and GAL-GFP-RZ persist at the nuclear periphery after transcription is repressed (blue and red;_Abruzzi et al., 2006). When the exosome is inactivated, the GAL-GFP-GALpA gene is no longer retained at the nuclear periphery after transcription shutoff (light blue). In contrast, the GAL-GFP-RZ reporter gene remains at the nuclear periphery for at least 60′ in the absence of transcription (pink). B) Message specific FISH was performed on cells expressing either GAL-GFP-GALpA or GAL-GFP-RZ in wild-type or rrp6Δ cells. Cells were fixed after growth in galactose (GAL) or various times after being shifted into glucose to repress transcription. As reported earlier, GAL-GFP-GALpA and GAL-GFP-RZ dots persist in the absence of transcription (dark blue and red; Abruzzi et al., 2006) When the exosome is inactivated by the deletion of RRP6, GAL-GFP-GALpA dots disappear rapidly while GAL-GFP-RZ dots remain stable (compare light blue and pink).

A) Diagram showing the steps used to purify and analyze the poly A- GAL1 mRNA via LM-3′RACE. In step 1, two biotinylated oligonucleotides complementary to the region at the beginning of the ORF (red) were used to isolate full length GAL1 mRNA. This total GAL1 mRNA was then passed through oligo dT resin (Ambion) to generate poly A- pool of GAL1 mRNA (Step 2). A 3′ blocked oligonucleotide with a 5′ P was ligated to the 3′ end of the selected RNA (Step 3; green), which was followed by RT-PCR for 3′ end analysis (Step 4; blue). B) LM-3′RACE analysis of poly A- pool of GAL1 mRNA. Total RNA isolated from cells after growth in glucose, galactose and various times after transcriptional repression were used to purify poly A- pool of GAL1 mRNA (see A). LM-3′Race was performed and the products were separated on a 2% agarose gel. In wild type cells we see a discrete band around the size of the first cleavage site (lanes 2, 3 and 4) that persists for >60 min after transcription shutoff. In rrp6Δ, this species is visible when transcription is active (lane 6) and disappears 15 min after transcriptional repression (lane 7). C) Sequencing analysis was performed on both the PCR products as well as the major band from the LM-3′RACE. Sequencing revealed an unpolyadenylated species of GAL1 mRNA cleaved precisely at the first cleavage site. The underlined sequence is the primer VS27.