The Bacillus subtilis spoOJ gene is required for accurate chromosome partitioning during growth and sporulation. We have characterized the subcellular localization of Spo0J protein by immunofluorescence and, in living cells, by use of a spoOJ-gfp fusion. We show that the Spo0J protein forms discrete stable foci usually located close to the cell poles. The foci replicate in concert with the initiation of new rounds of DNA replication, after which the daughter foci migrate apart inside the cell. This migration is independent of cell length extension, and presumably serves to direct the daughter chromosomes toward opposite poles of the cell, ready for division. During sporulation, the foci move to the extreme poles of the cell, where they function to position the oriC region of the chromosome ready for polar septation. These observations provide strong evidence for the existence of a dynamic, mitotic-like apparatus responsible for chromosome partitioning in bacteria. The mechanism by which bacterial chromosomes are equipartitioned into daughter cells at division has remained obscure despite decades of study (Hiraga 1993;Wake and Errington 1995). There are several reports of abrupt movement of bacterial nucleoids (Sargent 1974;Hiraga et al. 1990;Begg and Donachie 1991), suggesting the existence of an active partitioning machinery equivalent to the mitotic apparatus of eukaryotes. However, there are some difficulties in interpreting these results and van Helvoort and Woldringh (1994)have shown convincingly that unperturbed nucleoids move apart gradually and continuously during cell growth (van Helvoort and Woldringh 1994). The tendency to assume that the mechanisms of chromosome segregation in bacteria are distinct from those of eukaryotes stems mainly from the absence of obvious structures such as the cytoskeleton and the mitotic spindle. However, this might be attributable in part to the difficulty in resolving such structures in ceils as small and tough as bacteria. Detection of mitotic-like activity is also hampered by the relatively unstructured state of the bacterial nucleoid and, particu-

SummaryThe Spo0JA and Spo0JB proteins of Bacillus subtilis are similar to the ParA and ParB plasmid-partitioning proteins, respectively, and mutation of spo0JB prevents the expression of stage II genes of sporulation. This phenotype is a consequence of Spo0JA activity in the absence of Spo0JB, and its basis was unknown. In the studies reported here, Spo0JA was found specifically to dissociate transcription initiation complexes formed in vitro by the phosphorylated sporulation transcription factor Spo0A and RNA polymerase with the spoIIG promoter. This repressor-like activity is likely to be the basis for preventing the onset of differentiation in vivo. Spo0JB is known to neutralize Spo0JA activity in vivo and also to interact with a mitotic-like apparatus responsible for chromosome partitioning. These data suggest that Spo0JA and Spo0JB form a regulatory link between chromosome partition and development gene expression.

We have characterized the yyaA gene of Bacillus subtilis, located near the origin of chromosome replication (oriC). Its protein product is similar to the Spo0J protein, which belongs to the ParB family of chromosomeand plasmid-partitioning proteins. Insertional inactivation of the yyaA gene had no apparent effect on chromosome organization and partitioning during vegetative growth or sporulation. Subcellular localization of YyaA by immunofluorescence microscopy indicated that it colocalizes with the nucleoid, and gel retardation studies confirmed that YyaA binds relatively nonspecifically to DNA. Overexpression of yyaA caused a sporulation defect characterized by the formation of multiple septa within the cell. This phenotype indicates that YyaA may have a regulatory role at the onset of sporulation.