Srivastava Preeti, Fekete Richard A, Chattoraj Dhruba K
Laboratory of Biochemistry, NIH, Bethesda, MD 20892-4255, USA.
J Bacteriol. 2006 Feb;188(3):1060-70. doi: 10.1128/JB.188.3.1060-1070.2006.
Genome duplication and segregation normally are completed before cell division in all organisms. The temporal relation of duplication and segregation, however, can vary in bacteria. Chromosomal regions can segregate towards opposite poles as they are replicated or can stay cohered for a considerable period before segregation. The bacterium Vibrio cholerae has two differently sized circular chromosomes, chromosome I (chrI) and chrII, of about 3 and 1 Mbp, respectively. The two chromosomes initiate replication synchronously, and the shorter chrII is expected to complete replication earlier than the longer chrI. A question arises as to whether the segregation of chrII also is completed before that of chrI. We fluorescently labeled the terminus regions of chrI and chrII and followed their movements during the bacterial cell cycle. The chrI terminus behaved similarly to that of the Escherichia coli chromosome in that it segregated at the very end of the cell division cycle: cells showed a single fluorescent focus even when the division septum was nearly complete. In contrast, the single focus representing the chrII terminus could divide at the midcell position well before cell septation was conspicuous. There were also cells where the single focus for chrII lingered at midcell until the end of a division cycle, like the terminus of chrI. The single focus in these cells overlapped with the terminus focus for chrI in all cases. It appears that there could be coordination between the two chromosomes through the replication and/or segregation of the terminus region to ensure their segregation to daughter cells.
在所有生物体中，基因组复制和分离通常在细胞分裂之前完成。然而，在细菌中，复制和分离的时间关系可能会有所不同。染色体区域在复制时可以向相反的两极分离，或者在分离之前保持相当长一段时间的凝聚状态。霍乱弧菌有两条大小不同的环状染色体，分别是约3 Mbp的染色体I（chrI）和约1 Mbp的染色体II（chrII）。两条染色体同步启动复制，预计较短的chrII比较长的chrI更早完成复制。一个问题出现了，即chrII的分离是否也在chrI之前完成。我们用荧光标记了chrI和chrII的末端区域，并在细菌细胞周期中跟踪它们的移动。chrI末端的行为与大肠杆菌染色体的末端相似，即在细胞分裂周期的末期分离：即使分裂隔膜几乎完成，细胞也显示出一个单一的荧光焦点。相比之下，代表chrII末端的单一焦点在细胞隔膜明显出现之前就可以在细胞中部位置分开。也有一些细胞，chrII的单一焦点在细胞中部停留到分裂周期结束，就像chrI的末端一样。在所有情况下，这些细胞中的单一焦点都与chrI的末端焦点重叠。似乎两条染色体之间可能通过末端区域的复制和/或分离进行协调，以确保它们分离到子细胞中。
