SummaryWe have analysed the function of a gene of Bacillus subtilis, the product of which shows significant homology with eukaryotic SMC proteins essential for chromosome condensation and segregation. Two mutant strains were constructed; in one, the expression was under the control of the inducible spac promoter (conditional null) and, in the other, the gene was disrupted by insertion (disrupted null). Both could form colonies at 23ЊC but not at 37ЊC in the absence of the expression of the Smc protein, indicating that the B. subtilis smc gene was essential for cell growth at higher temperatures. Microscopic examination revealed the formation of anucleate and elongated cells and diffusion of nucleoids within the elongated cells in the disrupted null mutant grown at 23ЊC and in the conditional null mutant grown in low concentrations of IPTG at 37ЊC. In addition, immunofluorescence microscopy showed that subcellular localization of the Spo0J partition protein was irregular in the smc disrupted null mutant, compared with bipolar localization in wild-type cells. These results indicate that the B. subtilis smc gene is essential for chromosome partition. The role of B. subtilis Smc protein in chromosome partition is discussed.

We constructed Bacillus subtilis strains in which chromosome replication initiates from the minimal replicon of a plasmid isolated from Bacillus natto, independently of oriC. Integration of the replicon in either orientation at the proA locus (115؇ on the genetic map) suppressed the temperature-sensitive phenotype caused by a mutation in dnaA, a gene required for initiation of replication from oriC. In addition, in a strain with the plasmid replicon integrated into the chromosome, we were able to delete sequences required for oriC function. These strains were viable but had a slower growth rate than the oriC ؉ strains. Marker frequency analysis revealed that both pyrD and metD, genes close to proA, showed the highest values among the markers (genes) measured, and those of other markers decreased symmetrically with distance from the site of the integration (proA). These results indicated that the integrated plasmid replicon operated as a new and sole origin of chromosome replication in these strains and that the mode of replication was bidirectional. Interestingly, these mutants produced anucleate cells at a high frequency (about 40% in exponential culture), and the distribution of chromosomes in the cells was irregular. A change in the site and mechanism (from oriC to a plasmid system) of initiation appears to have resulted in a drastic alteration in coordination between chromosome replication and chromosome partition or cell division.

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Summary Current views of bacterial chromosome segregation vary in respect of the likely presence or absence of an active segregation mechanism involving a mitotic‐like apparatus. Furthermore, little is known about cis‐acting elements for chromosome segregation in bacteria. In this report, we show that two separate DNA regions, a 3′ coding region of dnaA and the AT‐rich sequence between dnaA and dnaN (the initial opening site of duplex DNA during replication), are necessary for efficient segregation of the chromosome in Bacillus subtilis. When a plasmid replicon was integrated into argG, far from oriC, on the chromosome and then the oriC function was disrupted, the oriC‐deleted mutant formed anucleate cells at 5% possibly because of defects in chromosome segregation. However, when the two DNA sequences were added near oriN, frequency of anucleate cells decreased to 1%. In these cells, the origin (argG) regions were localized near cell poles, whereas they were randomly distributed in cells without the two DNA sequences. These results suggest that the two DNA sequences in and downstream of the dnaA gene participate in correct positioning of the replication origin region within the cell and that this function is associated with accurate chromosome segregation in B. subtilis.

We constructed oriC-deleted mutants of Bacillus subtilis by integrating the minimal replication region of plasmid pLS32 into the proA (115 degrees), spoIIIJ (360 degrees) and thrS (256 degrees) loci of the chromosome, respectively. All three mutants produced anucleate cells and the DNA/protein ratio was lower than that of the wild-type strain when grown in nutrient broth. However, when grown in minimal-glucose medium, the frequency of anucleate cells was reduced in all of them and the DNA/protein ratio was restored to normal. Especially, the oriC-deleted mutant in which the plasmid was integrated near oriC produced almost no anucleate cell. These results indicate that initiation frequency of chromosome replication from the integrated plasmid origin were reduced disproportionately to cell mass increase in rich medium, which in turn disrupted coordination between DNA replication cycle and cell division cycle. The locations of the plasmid origin relative to the natural oriC locus affected the production of anucleate cell remarkably, suggesting that partition mechanism of chromosome was also impaired by the translocation of its replication origin.