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H L Withers

Publications and source records attributed to H L Withers.

5 recordsLinked to original sources
PubMed1998

Quorum-sensing acts at initiation of chromosomal replication in Escherichia coli.

Chromosomal replication in Escherichia coli was studied by flow cytometry and was found to be inhibited by an extracellular factor present in conditioned media collected during late exponential and early stationary phase, i.e., via a quorum-sensing mechanism. Our results suggest that the inhibitory activity of the extracellular factor is exerted during initiation of DNA replication rather than during elongation. Furthermore, we present evidence that this interaction may occur directly at each of the replication forks. Unlike other quorum-sensing systems described so far for Gram-negative bacteria, this inhibitory activity does not require transcription or translation to be effective. Implications of quorum-sensing regulation of DNA replication are discussed.

Chromosomes, Bacterial↗
PubMed1998

Characterization of dnaC2 and dnaC28 mutants by flow cytometry.

Escherichia coli strains containing thermosensitive dnaC alleles were studied by flow cytometry. Strains containing either the dnaC2 or dnaC28 allele were shifted between different temperatures, and DNA content distributions were gathered. Inhibition of initiation of chromosome replication at nonpermissive temperature, as well as reinitiation of replication at permissive temperature, were found to be affected by a number of parameters. These included the choice of permissive and nonpermissive temperatures, the length of the time of incubation at the nonpermissive temperature, the growth medium, the type of temperature shift used for reinitiation of replication (transient or nontransient), the genetic background of the host cell, and the cell concentration. Reinitiation of replication required neither transcription nor translation, whereas the elongation stage of replication was dependent upon ongoing protein synthesis in the mutants. Efficient use of dnaC mutants for cell cycle studies is discussed.

Bacterial Proteins↗
PubMed1993

Multicopy plasmid instability: the dimer catastrophe hypothesis.

Multimer formation reduces plasmid copy number and is an established cause of segregational instability. Nevertheless, it is difficult to rationalize observations that low levels of dimers can cause severe instability, if we assume they are distributed evenly in cell populations. We report here that dimer distribution is in fact heterogeneous in recombination-proficient strains. Most cells in the population contain only monomers; dimers are confined to a small subpopulation from which plasmid-free daughters arise at high frequency. In a rec+ culture where 4% of pBR322 molecules are dimers, more than half are in dimer-only cells. We show that this situation is inevitable because dimers replicate at twice the rate of monomers. Runaway multimerization is avoided because dimer-containing cells grow more slowly than their monomer-containing counterparts. A computer simulation is used to show how dimers proliferate after formation by homologous recombination. The equilibrium concentration of dimers is proportional to the inter-plasmid recombination rate and is essentially independent of the rate at which homologous recombination converts dimers to monomers.

Cell Division↗