Chromosome replication and cell division of Escherichia coli are coordinated with growth such that wild-type cells divide once and only once after each replication cycle. To investigate the nature of this coordination, the effects of inhibiting replication on Z-ring formation and cell division were tested in both synchronized and exponentially growing cells with only one replicating chromosome. When replication elongation was blocked by hydroxyurea or nalidixic acid, arrested cells contained one partially replicated, compact nucleoid located mid-cell. Cell division was strongly inhibited at or before the level of Z-ring formation. DNA cross-linking by mitomycin C delayed segregation, and the accumulation of about two chromosome equivalents at mid-cell also blocked Z-ring formation and cell division. Z-ring inhibition occurred independently of SOS, SlmA-mediated nucleoid occlusion, and MinCDE proteins and did not result from a decreased FtsZ protein concentration. We propose that the presence of a compact, incompletely replicated nucleoid or unsegregated chromosome masses at the normal mid-cell division site inhibits Z-ring formation and that the SOS system, SlmA, and MinC are not required for this inhibition. Bacterial DNA replication and cell division are coordinated with growth so that a single, timely division follows each genome duplication. This rule, however, is complicated by the fact that cells can divide rapidly, with the replication and division machinery operating continuously and concurrently (e.g., see reference 1). Division at mid-cell, which occurs with high precision (2, 3), begins with polymerization of the FtsZ protein into a circumferential ring on the cytoplasmic membrane inner surface (4). Although division frequency is ultimately determined by growth rate (5, 6), regulatory mechanisms control both the location and timing of FtsZ ring assembly (for recent reviews, see the works of de Boer [7], Chien et al. [8], Lutkenhaus et al. [9], and Egan and Vollmer [10]).Placement of Z rings mid-cell depends on negative activities of the Min proteins and nucleoid occlusion. The MinC protein of Escherichia coli binds to FtsZ, preventing Z-ring assembly at all positions except at mid-cell, where its time-averaged concentration is lowest (11-13). The nucleoid occlusion (14) proteins SlmA and Noc, of E. coli and Bacillus subtilis, respectively (15, 16), bind DNA at specific sequences and prevent division over replicating nucleoids in rapidly growing cells. SlmA prevents Z-ring formation by also binding FtsZ (17, 18); the Noc protein functions similarly but has not been shown to bind FtsZ directly (19). Because the SlmA and Noc DNA binding sites are absent from the terminus domain, which is replicated last and in the cell center (20-23), nucleoid occlusion contributes to both spatial and temporal control of Z-ring formation (17)(18)(19)). An slmA null mutant which is also min null frequently forms Z-ring-like structures over nucleoids when grown in LB medium (15). However, the SlmA protein might not be the ...

Clostridium taeniosporum, a non-pathogenic anaerobe closely related to the C. botulinum Group II members, was isolated from Crimean lake silt about 60 years ago. Its endospores are surrounded by an encasement layer which forms a trunk at one spore pole to which about 12–14 large, ribbon-like appendages are attached. The genome consists of one 3,264,813 bp, circular chromosome (with 26.6% GC) and three plasmids. The chromosome contains 2,892 potential protein coding sequences: 2,124 have specific functions, 147 have general functions, 228 are conserved but without known function and 393 are hypothetical based on the fact that no statistically significant orthologs were found. The chromosome also contains 101 genes for stable RNAs, including 7 rRNA clusters. Over 84% of the protein coding sequences and 96% of the stable RNA coding regions are oriented in the same direction as replication. The three known appendage genes are located within a single cluster with five other genes, the protein products of which are closely related, in terms of sequence, to the known appendage proteins. The relatedness of the deduced protein products suggests that all or some of the closely related genes might code for minor appendage proteins or assembly factors. The appendage genes might be unique among the known clostridia; no statistically significant orthologs were found within other clostridial genomes for which sequence data are available. The C. taeniosporum chromosome contains two functional prophages, one Siphoviridae and one Myoviridae, and one defective prophage. Three plasmids of 5.9, 69.7 and 163.1 Kbp are present. These data are expected to contribute to future studies of developmental, structural and evolutionary biology and to potential industrial applications of this organism.