Pogliano J, Pogliano K, Weiss D S, Losick R, Beckwith J
Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, MA 02115, USA.
Proc Natl Acad Sci U S A. 1997 Jan 21;94(2):559-64. doi: 10.1073/pnas.94.2.559.
A universally conserved event in cell division is the formation of a cytokinetic ring at the future site of division. In the bacterium Escherichia coli, this ring is formed by the essential cell division protein FtsZ. We have used immunofluorescence microscopy to show that FtsZ assembles early in the division cycle, suggesting that constriction of the FtsZ ring is regulated and supporting the view that FtsZ serves as a bacterial cytoskeleton. Assembly of FtsZ rings was heterogeneously affected in an ftsI temperature-sensitive mutant grown at the nonpermissive temperature, some filaments displaying a striking defect in FtsZ assembly and others displaying little or no defect. By using low concentrations of the beta-lactams cephalexin and piperacillin to specifically inhibit FtsI (PBP3), an enzyme that synthesizes peptidoglycan at the division septum, we show that FtsZ ring constriction requires the transpeptidase activity of FtsI. Unconstricted FtsZ rings are stably trapped at the midpoint of the cell for several generations after inactivation of FtsI, whereas partially constricted FtsZ rings are less effectively trapped. In addition, FtsZ rings are able to assemble in newborn cells in the presence of cephalexin, suggesting that newborn cells contain a site at which FtsZ can assemble (the nascent division site) and that the transpeptidase activity of FtsI is not required for assembly of FtsZ at these sites. However, aside from this first round of FtsZ ring assembly, very few additional FtsZ rings assemble in the presence of cephalexin, even after several generations of growth. One interpretation of these results is that the transpeptidase activity of FtsI is required, directly or indirectly, for the assembly of nascent division sites and thereby for future assembly of FtsZ rings.
细胞分裂中一个普遍保守的事件是在未来分裂位点形成细胞分裂环。在细菌大肠杆菌中，这个环由必需的细胞分裂蛋白FtsZ形成。我们利用免疫荧光显微镜观察发现，FtsZ在分裂周期早期组装，这表明FtsZ环的收缩是受调控的，也支持了FtsZ作为细菌细胞骨架的观点。在非允许温度下生长的ftsI温度敏感突变体中，FtsZ环的组装受到异质性影响，一些细丝在FtsZ组装中表现出明显缺陷，而另一些则几乎没有缺陷。通过使用低浓度的β-内酰胺类药物头孢氨苄和哌拉西林来特异性抑制FtsI（PBP3），一种在分裂隔膜处合成肽聚糖的酶，我们发现FtsZ环的收缩需要FtsI的转肽酶活性。FtsI失活后，未收缩的FtsZ环会在细胞中点稳定滞留几代，而部分收缩的FtsZ环则较难被滞留。此外，在头孢氨苄存在的情况下，FtsZ环能够在新生细胞中组装，这表明新生细胞含有一个FtsZ能够组装的位点（新生分裂位点），并且在这些位点FtsZ的组装不需要FtsI的转肽酶活性。然而，除了第一轮FtsZ环组装外，即使经过几代生长，在头孢氨苄存在的情况下也很少有额外的FtsZ环组装。这些结果的一种解释是，FtsI的转肽酶活性直接或间接地是新生分裂位点组装所必需的，从而也是未来FtsZ环组装所必需的。
