Bacillus subtilis SMC is required for proper arrangement of the chromosomeand for efficient segregation of replication termini but not for bipolar movement of newly duplicated origin regions

SMC protein is required for chromosome condensation and for the faithful se gregation of daughter chromosomes in Bacillus subtilis. The visualization o f specific sites on the chromosome showed that newly duplicated origin regi ons in growing cells of an sme mutant were able to segregate from each othe r but that the location of origin regions was frequently aberrant. In contr ast, the segregation of replication termini was impaired in sme mutant cell s. This analysis was extended to germinating spores of an sme mutant. The r esults showed that during germination, newly duplicated origins, but not te rmini, were able to separate from each other in the absence of SMC. Also, D API (4',6'-diamidino-2-phenylindole) staining revealed that chromosomes in germinating spores were able to undergo partial or complete replication but that the daughter chromosomes were blocked at a late stage in the segregat ion process. These findings were confirmed by time-lapse microscopy, which showed that after duplication in growing cells the origin regions underwent rapid movement toward opposite poles of the cell in the absence of SMC. Th is indicates that SMC is not a required component of the mitotic motor that initially drives origins apart after their duplication. It is also conclud ed that SMC is needed to maintain the proper layout of the chromosome in th e cell and that it functions in the cell cycle after origin separation but prior to complete segregation or replication of daughter chromosomes. It is proposed here that chromosome segregation takes place in at least two step s: an SMC-independent step in which origins move apart and a subsequent SMC -dependent step in which newly duplicated chromosomes condense and are ther eby drawn apart.