SummaryUsing a bacterial two-hybrid system and a combination of in vivo and in vitro assays that take advantage of the green fluorescent reporter protein (GFP), we have investigated the localization and the proteinprotein interaction of several key components of the cytokinetic machinery of cyanobacteria (i.e. the progenitor of chloroplast). We demonstrate that (i) the ftsZ and zipN genes are essential for the viability of the model cyanobacterium Synechocystis sp. PCC 6803, whereas the minCDE cluster is dispensable for cell growth; (ii) the GTP-binding domain of FtsZ is crucial to FtsZ assembly into the septal ring at midcell; (iii) the Z-ring of deeply constricted daughter cells is oriented perpendicularly to the mother Z-ring, showing that Synechocystis divides in alternating perpendicular planes; (iv) the MinCDE system affects the morphology of the cell, as well as the position and the shape of FtsZ structures; and (v) MinD is targeted to cell membranes in a process involving its Cterminal amphipathic helix, but not its ATP-binding region. Finally, we have also characterized a novel Zinteracting protein, ZipN, the N-terminal DnaJ domain of which is critical to the decoration of the Z-ring, and we report that this process is independent of MinCDE.
SummaryThe cyanobacterial genes lexA , recA and ruvB were analysed in Synechocystis PCC6803, which is shown here to be more radiation resistant than the other unicellular model strain Synechococcus PCC7942. We found that cyanobacteria do not have an Escherichia coli -type SOS regulon. The Synechocystis lexA and recA promoters were found to be strong and UV insensitive, unlike the ruvB promoter, which is weak and UV-C inducible. Yet, lexA and recA are regulated by UV-C, but the control is negative and occurs at the post-transcriptional level. Two novel conserved elements were characterized in the lexA promoter: (i) an unusually long crucial box 5 ¢ ¢ ¢ ¢ -TAAAATTTTGTATCTTTT-3 ¢ ¢ ¢ ¢ ( ----64, ----47); and (ii) a negatively acting motif 5 ¢ ¢ ¢ ¢ -TAT GAT-3 ¢ ¢ ¢ ¢ ( ----42, ----37). These elements were not found in the recA promoter, which appeared to be unusually simple in harbouring only a single crucial element (i.e. the canonical ----10 box). RuvB, operating in recombination-dependent cellular processes, was found to be dispensable to cell growth, whereas LexA and RecA appeared to be critical to cell viability. Using DNA microarrays, we have identified 57 genes with expression that is altered, at least twofold, in response to LexA depletion. None of these genes is predicted to operate in DNA metabolism, arguing against the involvement of LexA in the regulation of DNA repair. Instead, most of the LexA-responsive genes were known to be involved in carbon assimilation or controlled by carbon availability. Consistently, the growth of the LexA-depleted strain was found to be strongly dependent on the availability of inorganic carbon.
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