Possible role for the essential GTP-binding protein Obg in regulating the initiation of sporulation in Bacillus subtilis.

We fused obg, encoding an essential GTP-binding protein in Bacillus subtilis, to the LacI-repressible, IPTG (isopropyl-beta-D-thiogalactopyranoside)-inducible promoter Pspac. Depletion of Obg, following removal of IPTG, caused a defect in sporulation and in expression of sporulation genes that are activated by Spo0A approximately P. These defects were significantly relieved by a mutation in spo0A (rvtA11) that bypasses the normal phosphorylation pathway, indicating that Obg might normally be required, either directly or indirectly, to stimulate activity of the phosphorelay that activates Spo0A.     

Phosphatases modulate the bistable sporulation gene expression pattern in Bacillus subtilis

Summary Spore formation in the Gram-positive bacterium Bacillus subtilis is a last resort adaptive response to starvation. To initiate sporulation, the key regulator in this process, Spo0A, needs to be activated by the so-called phosphorelay. Within a sporulating culture of B. subtilis, some cells initiate this developmental program, while other cells do not. Therefore, initiation of sporulation appears to be a regulatory process with a bistable outcome. Using a single cell analytical approach, we show that the autostimulatory loop of spo0A is responsible for generating a bistable response resulting in phenotypic variation within the sporulating culture. It is demonstrated that the main function of RapA, a phosphorelay phosphatase, is to maintain the bistable sporulation gene expression. As rapA expression is quorum regulated, it follows that quorum sensing influences sporulation bistability. Deletion of spo0E, a phosphatase directly acting on Spo0A approximately P, resulted in abolishment of the bistable expression pattern. Artificial induction of a heterologous Rap phosphatase restored heterogeneity in a rapA or spo0E mutant. These results demonstrate that with external phosphatases, B. subtilis can use the phosphorelay as a tuner to modulate the bistable outcome of the sporulating culture. This shows that B. subtilis employs multiple pathways to maintain the bistable nature of a sporulating culture, stressing the physiological importance of this phenomenon.     

AbrB and Spo0E Control the Proper Timing of Sporulation in <Emphasis Type="Italic">Bacillus subtilis</Emphasis>

We have shown previously that Spo0AP-dependent sinIR operon expression was substantially down-regulated in abrB null mutant backgrounds. In this report, we show that loss of function mutations in abrB also cause phosphorelay gene expression to be down regulated. abrB null mutations caused diminished vegetative growth-associated sporulation and resulted in a significant reduction in sporulation frequencies at T₂₄. These mutants, however, sporulated at wild-type levels at T₄₈, indicating that sporulation timing was affected. The rvtA11 mutation in spo0A, a deletion mutation in spo0E, and a null mutation in hpr (scoC) rescued sporulation and Spo0AP-dependent gene expression in an abrB mutant background. These data indicate that AbrB and Spo0E may comprise a checkpoint system that regulates the progression of sporulation, allowing exploration of alternate cell states prior to the irrevocable commitment to sporulation.     

Effects of the chromosome partitioning protein Spo0J (ParB) on oriC positioning and replication initiation in Bacillus subtilis

Spo0J (ParB) of Bacillus subtilis is a DNA-binding protein that belongs to a conserved family of proteins required for efficient plasmid and chromosome partitioning in many bacterial species. We found that Spo0J contributes to the positioning of the chromosomal oriC region, but probably not by recruiting the origin regions to specific subcellular locations. In wild-type cells during exponential growth, duplicated origin regions were generally positioned around the cell quarters. In a spo0J null mutant, sister origin regions were often closer together, nearer to midcell. We found, by using a Spo0J-green fluorescent protein [GFP] fusion, that the subcellular location of Spo0J was a consequence of the chromosomal positions of the Spo0J binding sites. When an array of binding sites (parS sites) were inserted at various chromosomal locations in the absence of six of the eight known parS sites, Spo0J-GFP was no longer found predominantly at the cell quarters, indicating that Spo0J is not sufficient to recruit chromosomal parS sites to the cell quarters. spo0J also affected chromosome positioning during sporulation. A spo0J null mutant showed an increase in the number of cells with some origin-distal regions located in the forespore. In addition, a spo0J null mutation caused an increase in the number of foci per cell of LacI-GFP bound to arrays of lac operators inserted in various positions in the chromosome, including the origin region, an increase in the DNA-protein ratio, and an increase in origins per cell, as determined by flow cytometry. These results indicate that the spo0J mutant produced a significant proportion of cells with increased chromosome content, probably due to increased and asynchronous initiation of DNA replication.     

In Vivo Effects of Sporulation Kinases on Mutant Spo0A Proteins in Bacillus subtilis

The phosphorylated form of the response regulator Spo0A (Spo0A~P) is required for the initiation of sporulation in Bacillus subtilis. Phosphate is transferred to Spo0A from at least four histidine kinases (KinA, KinB, KinC, and KinD) by a phosphotransfer pathway composed of Spo0F and Spo0B. Several mutations in spo0A allow initiation of sporulation in the absence of spo0F and spo0B, but the mechanisms by which these mutations allow bypass of spo0F and spo0B are not fully understood. We measured the ability of KinA, KinB, and KinC to activate sporulation of five spo0A mutants in the absence of Spo0F and Spo0B. We also determined the effect of Spo0E, a Spo0A~P-specific phosphatase, on sporulation of strains containing the spo0A mutations. Our results indicate that several of the mutations relax the specificity of Spo0A, allowing Spo0A to obtain phosphate from a broader group of phosphodonors. In the course of these experiments, we observed medium-dependent effects on the sporulation of different mutants. This led us to identify a small molecule, acetoin, that can stimulate sporulation of some spo0A mutants.