Many of the proteins that operate at the replication fork in Escherichia coli have been defined genetically. These include some of the subunits of the DNA polymerase MI holoenzyme, the DnaB replication fork helicase, and the DnaG primase. The multiprotein primosome (which includes the DnaB and DnaG proteins), defined biochemically on the basis of its requirement during bacteriophage 4vX174 complementary-strand synthesis, could serve as the helicase-primase replication machine on the lagging-strand template. In order to determine if this is the case, we have begun an investigation of the phenotypes of mutants with mutations pri4, priB, and priC, which encode the primosomal proteins factor Y (protein n'), n, and n", respectively. Inactivation ofpri4 by insertional mutagenesis resulted in the induction of the SOS response, as evinced by induction of a resident X prophage, extreme filamentation, and derepression of an indicator operon in which j-galactosidase production was controlled by the dinDI promoter. In addition, the copy numbers of resident pBR322 plasmids were reduced four-to fivefold in these strains, and production of 4X174 phage was delayed considerably. These results are discussed in the context of existing models for SOS induction and possible roles for the PriA protein at the replication fork in vivo.

The role of PriA, required for the assembly of the X174-type primosome on DNA, in cellular DNA replication has been unclear since its discovery. Recent evidence, based on the phenotypes of strains carrying priA null mutations, has led to proposals that the primosome assembly activity of PriA was required to load replication forks at intermediates such as D loops during homologous recombination.

topB, encoding topoisomerase III, was identified as a high copy suppressor of the temperature-sensitive parC1215 allele, encoding one of the subunits of topoisomerase IV. Overexpression of topoisomerase III at the nonpermissive temperature was shown subsequently to restore timely chromosome decatenation and suppress lethality in strains carrying either temperature-sensitive parE or parC alleles. By developing an assay in vitro for precatenane unlinking, we demonstrated directly that both topoisomerase III and topoisomerase IV were efficient at this task, whereas DNA gyrase was very inefficient at precatenane removal. These observations suggest that precatenane unlinking is sufficient to sustain decatenation of replicating daughter chromosomes in the cell.

Background: MreB, a bacterial actin homolog, is required for the maintenance of cell shape in E. coli. Results: E. coli MreB has been purified, and its polymerization has been characterized. Conclusion: E. coli MreB is capable of forming filament bundles in vitro.Significance: The ability to investigate E. coli MreB function in vitro will help answer significant questions about its function in vivo.

Escherichia coli replication factor Y (protein n') functions in the assembly of a mobile multiprotein replication-priming complex called the primosome. Although the role of factor Y in primosome assembly during replication in vitro of bacteriophage 4X174 and plasmid pBR322 DNA is clear, its role in E. coli chromosomal replication is not.

The properties of two mutant PriA proteins, PriA C439Y and PriA C445Y have been used to reveal the role of PriB during assembly of the phiX174-type primosome. The replication defects of both mutant PriA proteins could be rescued by high concentrations of DnaT. Analysis of the formation of intermediate complexes in primosome assembly and the effect of PriB on PriA binding to DNA demonstrated that the mutant PriA proteins could not form a PriA-PriB complex on DNA carrying a primosome assembly site. Consequently, the mutant proteins also could not form PriA-PriB-DnaT complexes at concentrations of DnaT sufficient to form such a complex with wild-type PriA. In addition, PriB was found to stabilize wild-type but not mutant PriA proteins on DNA. At high concentrations of DnaT, both mutant and wild-type PriA proteins could form a PriA-DnaT complex and support PriB-independent phiX174 complementary strand DNA replication. Thus, during primosome assembly, PriB facilitates complex formation between PriA and DnaT.

MukB is a structural maintenance of chromosome-like protein required for DNA condensation. The complete condensin is a large tripartite complex of MukB, the kleisin, MukF, and an accessory protein, MukE. As found previously, MukB DNA condensation is a stepwise process. We have defined these steps topologically. They proceed first via the formation of negative supercoils that are sequestered by the protein followed by hinge-hinge interactions between MukB dimers that stabilize topologically isolated loops in the DNA. MukB itself is sufficient to mediate both of these topological alterations; neither ATP nor MukEF is required. We show that the MukB hinge region binds DNA and that this region of the protein is involved in sequestration of supercoils. Cells carrying mutations in the MukB hinge that reduce DNA condensation exhibit nucleoid decondensation.

SummarysetB was identified as a high-copy suppressor of the partition defect of a mutation in parC , encoding one of the subunits of topoisomerase IV. Deletion of this integral inner membrane protein causes a delay in chromosome segregation, whereas its overproduction causes nucleoid disintegration and stretching, leading to a cell division defect. setB deletion mutants also exhibit a synthetic phenotype when combined with mutations that delete the C-terminal motor domain of the septal ring protein FtsK. SetB localizes in the cell as a helix and interacts with MreB, the bacterial actin homologue, which also forms a helix. These observations suggest that there may be a link between chromosome segregation and cellular infrastructure.