Graumann P L
Department of Molecular and Cellular Biology, The Biological Laboratories, Harvard University, Cambridge, Massachusetts 02138, USA.
J Bacteriol. 2000 Nov;182(22):6463-71. doi: 10.1128/JB.182.22.6463-6471.2000.
SMC protein is required for chromosome condensation and for the faithful segregation of daughter chromosomes in Bacillus subtilis. The visualization of specific sites on the chromosome showed that newly duplicated origin regions in growing cells of an smc mutant were able to segregate from each other but that the location of origin regions was frequently aberrant. In contrast, the segregation of replication termini was impaired in smc mutant cells. This analysis was extended to germinating spores of an smc mutant. The results showed that during germination, newly duplicated origins, but not termini, were able to separate from each other in the absence of SMC. Also, DAPI (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 segregation 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. This indicates that SMC is not a required component of the mitotic motor that initially drives origins apart after their duplication. It is also concluded that SMC is needed to maintain the proper layout of the chromosome in the 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 steps: an SMC-independent step in which origins move apart and a subsequent SMC-dependent step in which newly duplicated chromosomes condense and are thereby drawn apart.
SMC蛋白对于枯草芽孢杆菌中的染色体凝聚以及子染色体的准确分离是必需的。对染色体上特定位点的可视化观察表明，在smc突变体生长细胞中新复制的起始区域能够相互分离，但起始区域的位置常常异常。相比之下，smc突变体细胞中复制终点的分离受到损害。该分析扩展到了smc突变体的萌发孢子。结果表明，在萌发过程中，新复制的起始点而非终点，在没有SMC的情况下能够相互分离。此外，4',6'-二脒基-2-苯基吲哚（DAPI）染色显示，萌发孢子中的染色体能够进行部分或完全复制，但子染色体在分离过程的后期被阻断。延时显微镜观察证实了这些发现，其显示在生长细胞中复制后，在没有SMC的情况下，起始区域迅速向细胞的两极移动。这表明SMC不是在复制后最初驱动起始点分开的有丝分裂动力的必需组成部分。还得出结论，SMC对于维持染色体在细胞中的正确布局是必需的，并且它在细胞周期中起始点分离后但在子染色体完全分离或复制之前发挥作用。这里提出染色体分离至少分两步进行：第一步是起始点分开的不依赖SMC的步骤，第二步是新复制的染色体凝聚从而被拉开的依赖SMC的后续步骤。
