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1.
Fig. 6

Fig. 6. Extracellular AI-2 activity in co-cultures of S. meliloti with E. carotovora. From: Sinorhizobium meliloti, a bacterium lacking the autoinducer-2 synthase, responds to AI-2 supplied by other bacteria.

Extracellular AI-2 activity was measured in a pure culture of E. carotovora WT strain SCC3193 (triangles) or co-cultures of the following combinations: S. meliloti (WT) with E. carotovora WT strain SCC3193 (circles),S. meliloti (WT) with E. carotovora luxS strain SCC6023 (crosses), and S. meliloti aitK strain MET2002 with E. carotovora WT strain SCC3193 (squares). All strains were grown in LBMC medium. AI-2 activity in the cell-free culture fluids is reported as fold induction of light production by V. harveyi BB170.

Catarina S. Pereira, et al. Mol Microbiol. ;70(5):1223-1235.
2.
Fig. 2

Fig. 2. Binding of AI-2 to potential receptor proteins. From: Sinorhizobium meliloti, a bacterium lacking the autoinducer-2 synthase, responds to AI-2 supplied by other bacteria.

Light produced by V. harveyi strain MM32 (LuxN, LuxS) was assayed following the addition of ligand released from purified protein expressed in either LuxS+ (black bars) or LuxS (white bars) E. coli (strains BL21 and FED101, respectively). The E. coli ribose binding protein RbsB and protein-free GSH-buffer were included as negative controls. AI-2 activity is reported as c.p.s. of MM32 bioluminescence. Error bars represent the standard deviations for three independent cultures.

Catarina S. Pereira, et al. Mol Microbiol. ;70(5):1223-1235.
3.
Fig. 3

Fig. 3. Structure of S. meliloti LsrB and its ligand. From: Sinorhizobium meliloti, a bacterium lacking the autoinducer-2 synthase, responds to AI-2 supplied by other bacteria.

A. Ribbon diagram of SmLsrB. The ribbon diagram is colored in rainbow order from N- to C- terminus. The bound ligand and the corresponding electron density are shown.
B. Stereoview of 2Fo-Fc ligand electron density. The DPD isomer R-THMF is shown modeled into non-protein electron density in the receptor binding site.
C. Comparison of the of the S. meliloti and S. typhimurium LsrB AI-2 binding sites. Overlay of the SmLsrB (green) and S. typhimurium LsrB (yellow) binding sites based on overall alignment of the protein structures as calculated by PyMOL. Residue numbers are from the S. meliloti sequence. Dashed red lines indicate potential hydrogen bonds and the interacting residues are labeled in red.

Catarina S. Pereira, et al. Mol Microbiol. ;70(5):1223-1235.
4.
Fig. 5

Fig. 5. Extracellular AI-2 activity in S. meliloti ait mutants. From: Sinorhizobium meliloti, a bacterium lacking the autoinducer-2 synthase, responds to AI-2 supplied by other bacteria.

Cultures of the following S. meliloti strains, RM1021 (WT, diamonds), MET2000 (aitA, squares) and MET2002 (aitK, triangles), were grown in LBMC with in vitro synthesized AI-2. AI-2 activity in the cell-free culture fluids is reported as fold induction of light production by V. harveyi BB170 (solid lines) and cell growth was monitored by optical density (dashed lines). The average optical density of the three cultures is shown (circles) with the corresponding standard deviation. Fold inductions above 1000 correspond to AI-2 concentrations that saturate the V. harveyi bioassay and are highlighted by the grey shadow

Catarina S. Pereira, et al. Mol Microbiol. ;70(5):1223-1235.
5.
Fig. 4

Fig. 4. S. meliloti internalization of exogenously supplied AI-2. From: Sinorhizobium meliloti, a bacterium lacking the autoinducer-2 synthase, responds to AI-2 supplied by other bacteria.

A. Extracellular AI-2 activity in S. meliloti cultures. Wild-type S. meliloti Rm1021 was cultured in LBMC in the presence (triangles) and absence (squares) of in vitro synthesized AI-2 and aliquots were taken at the specified times. AI-2 activity in cell-free culture fluids is reported as fold induction of light production by V. harveyi BB170 (solid lines). Samples with fold inductions above 1000 were diluted in LBMC and values shown were calculated taking account the dilution factor. Cell growth was monitored by optical density (dashed lines).
B. Expression of the S. meliloti aitB transcript in the presence and absence of AI-2. RNA levels in cultures of wild-type S. Meliloti RM1021 grown in the presence (black bars) and absence (white bars) of in vitro synthesized AI-2 were measured using quantitative real-time PCR. aitB transcript levels are reported as fold increase of aitB transcript in relation to the rpsL transcript. Cell growth in the presence (black circles) and in the absence (white circles) of AI-2 was measured by optical density.

Catarina S. Pereira, et al. Mol Microbiol. ;70(5):1223-1235.
6.
Fig. 1

Fig. 1. The interconversion of DPD into the known AI-2 ligands and the AI-2-dependent internalization system. From: Sinorhizobium meliloti, a bacterium lacking the autoinducer-2 synthase, responds to AI-2 supplied by other bacteria.

A. Proposed equilibrium between the currently known forms of AI-2 and their common precursor, DPD. The V. harveyi and S. typhimurium ligands have previously been shown to interconvert in solution.
B. S. typhimurium and E. coli Lsr-mediated transport and processing of AI-2. In S. typhimurium and E. coli, AI-2 is produced within the cell by LuxS and is secreted to the medium. As the concentration of extracellular AI-2 increases, AI-2 binds to the periplasmic binding protein LsrB and is internalized by the Lsr system, an ABC-type transport system. Once in the cytoplasm, AI-2 is phosphorylated (P-AI-2) by LsrK. P-AI-2 binds to the repressor of the lsr operon, LsrR, inactivating LsrR, relieving repression, and inducing transcription of lsr. This causes a rapid increase in the production of the Lsr transporter and, consequently removal of AI-2 from the environment. P-AI-2 is further processed by a mechanism not fully understood involving LsrG and LsrF. The lsrE gene is also present in the operon of S. typhimurium but not in E. coli and its function is not known.
C. The lsr orthologues in S. meliloti operon. S. meliloti has orthologs to all the genes of the lsr operon except lsrE. We named the S. meliloti lsr-like operon ait (for autoinducer transporter). The percent identity to the Lsr proteins from S. typhimurium is shown.

Catarina S. Pereira, et al. Mol Microbiol. ;70(5):1223-1235.

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