Direct procedure for the determination of the number of replication forks and the reinitiation fraction in bacteria

A direct method for calculating the average number of replication forks per chromosome in an exponentially growing bacterial culture and the fraction of reinitiation after an inductive treatment of the initiation step is presented. This method has allowed the development of REPLICON, a computer program designed for the resolution of the algorithm and simulation of the bacterial chromosome replication. Using REPLICON the following parameters can be obtained: average number of replication forks per chromosome, time required for the complete replication cycle, average amount of DNA per nucleotide, gene frequency of any chromosomal locus and reinitiation fraction. The use of this analysis also permits the determination of the uni- or bidirectionality of replication.

Differential inhibition of the initiation of DNA replication in stringent and relaxed strains of Escherichia coli

Starvation for isoleucine inhibits chromosome, minichromosome and pBR322 DNA replication in a stringent strain ofE. coli, but does not do so in a relaxed mutant. Starvation for other amino acids inhibits either chromosome and minichromosome replication in both strains. From these results we conclude thatoriCand pBR322 replication are stringently regulated and that isoleucine seems not to be essential for the protein synthesis required at the initiation oforiCreplication. Deprivation of isoleucine in a Rel−strain gives rise to amplification of minichromosome and pBR322 with a better yield of the latter plasmid than that following treatment with chloramphenicol.

Heat stress in the presence of low RNA polymerase activity increases chromosome copy number of Escherichia coli

A temperature shift-up accompanied by a reduction in RNA polymerase activity in Escherichia coli causes an increased rate of initiation leading to a 1.7- to 2.2-fold increase in chromosome copy number. A temperature shift-up without a reduction in polymerase activity induces only a transient non-scheduled initiation of chromosome replication caused by heat shock with no detectable effect on chromosome copy number.

Effect of multiple copies of rpoBC on the rate of RNA synthesis in Escherichia coli

The cloning of therpoBandrpoCgenes in a high copy number vector inE. coliincreased the amount of the encoded gene products, the β and β′ subunits of RNA polymerase. However, this unexpectedly caused a 30–50% decrease in RNA synthetic activity which alternatively induced a reduction of growth rate and enlargement of cell size, and decreased the DNA replication time. The results can be explained by autogenous regulation of the RNA polymerase genes by the ββ′ subunits. A relation between the decrease in number of transcription units and the observed higher rate of movement of DNA replication forks is discussed.