Bach Trond, Skarstad Kirsten
Department of Cell Biology, Institute for Cancer Research, The Norwegian Radium Hospital, 0310 Oslo, Norway.
Mol Microbiol. 2004 Mar;51(6):1589-600. doi: 10.1111/j.1365-2958.2003.03943.x.
In rapidly growing Escherichia coli cells replication cycles overlap and initiation occurs at multiple replication origins (oriCs). All origins within a cell are initiated essentially in synchrony and only once per cell cycle. Immediate re-initiation of new origins is avoided by sequestration, a mechanism dependent on the SeqA protein and Dam methylation of GATC sites in oriC. Here, GATC sites in oriC were changed to GTTC. This reduced the sequestration to essentially the level found in SeqA-less cells. The mutant origins underwent re-initiation, showing that the GATC sites in oriC are required for sequestration. Each re-initiation eventually gave rise to a cell containing an extra nucleoid. The three-nucleoid cells displayed one asymmetrically placed FtsZ-ring and divided into a two-nucleoid cell and a one-nucleoid cell. The three nucleoid-cells thus divided into three daughters by two consecutive divisions. The results show that extra rounds of replication cause extra daughter cells to be formed prematurely. The fairly normal mutant growth rate and size distribution show, however, that premature rounds of replication, chromosome segregation, and cell division are flexibly accommodated by the existing cell cycle controls.
在快速生长的大肠杆菌细胞中,复制周期相互重叠,并且在多个复制起点(oriC)处发生起始。细胞内的所有起点基本上同步起始,且每个细胞周期仅起始一次。通过隔离机制可避免新起点的立即重新起始,该机制依赖于SeqA蛋白和oriC中GATC位点的Dam甲基化。在此,oriC中的GATC位点被改变为GTTC。这将隔离作用降低到了基本上与缺乏SeqA的细胞中所发现的水平。突变的起点经历了重新起始,表明oriC中的GATC位点是隔离所必需的。每次重新起始最终都会产生一个含有额外类核的细胞。具有三个类核的细胞显示出一个不对称放置的FtsZ环,并分裂为一个含有两个类核的细胞和一个含有一个类核的细胞。因此,具有三个类核的细胞通过连续两次分裂产生三个子代细胞。结果表明,额外的复制轮次会导致额外的子代细胞过早形成。然而,相当正常的突变体生长速率和大小分布表明,现有的细胞周期调控机制能够灵活地适应过早的复制轮次、染色体分离和细胞分裂。