SummaryThe genes involved in flagellum synthesis, motility and chemotaxis in Escherichia coli are expressed in a hierarchical fashion. At the top of the hierarchy lies the master regulator FlhDC, required for the expression of the whole set of genes. The operon flhDC is controlled by numerous regulators including H-NS, CRP, EnvZ/OmpR, QseBC and LrhA. In the present work, we report that the flhDC operon is also negatively regulated by the His-Asp phosphorelay system RcsCDB. The regulation is potentiated by the RcsB cofactor RcsA. Genetic analysis indicates that an RcsAB box, located downstream of the promoter, is required for the regulation. The binding of RcsB and RcsA to this site was demonstrated by gel retardation and DNase I protection assays. In addition, mutation analysis suggests that RcsA-specific determinants lie in the right part of the 'RcsAB box'.

Most bacterial chromosomes carry an analogue of the parABS systems that govern plasmid partition, but their role in chromosome partition is ambiguous. parABS systems might be particularly important for orderly segregation of multipartite genomes, where their role may thus be easier to evaluate. We have characterized parABS systems in Burkholderia cenocepacia, whose genome comprises three chromosomes and one low-copynumber plasmid. A single parAB locus and a set of ParB-binding (parS) centromere sites are located near the origin of each replicon. ParA and ParB of the longest chromosome are phylogenetically similar to analogues in other multichromosome and monochromosome bacteria but are distinct from those of smaller chromosomes. The latter form subgroups that correspond to the taxa of their hosts, indicating evolution from plasmids. The parS sites on the smaller chromosomes and the plasmid are similar to the "universal" parS of the main chromosome but with a sequence specific to their replicon. In an Escherichia coli plasmid stabilization test, each parAB exhibits partition activity only with the parS of its own replicon. Hence, parABS function is based on the independent partition of individual chromosomes rather than on a single communal system or network of interacting systems. Stabilization by the smaller chromosome and plasmid systems was enhanced by mutation of parS sites and a promoter internal to their parAB operons, suggesting autoregulatory mechanisms. The small chromosome ParBs were found to silence transcription, a property relevant to autoregulation.

SummaryEscherichia coli FtsK is a large 1329 aa integral membrane protein, which links cell division and chromosome segregation through the respective activities of its 200 aa amino-terminal domain, FtsKN, and its 500 aa carboxy-terminal domain, FtsKC. A long 600 aa linker, FtsKL, connects these two domains. Only FtsKN is essential for cell division. However, previous observations suggested that the cytoplasmic part of FtsK also participates in the process of septation. Here, we identify two distinct regions within FtsKL, FtsK179-331 and FtsK332-641, which together with FtsKN, are required for normal septation. We discuss how the implication of multiple regions along the FtsK protein in cell division could participate in the co-ordination of this process with the last stages of chromosome segregation.