Studies with extrachromosomal particles in Escherichia coli K-12.

The reported phenomenon that the F factor was eliminated from the cell under conditions of thymine starvation, was studied because this seemed to be a suitable system to study certain aspects of the control of DNA replication. However, it was found that thymine starvation does not result in the elimination of the F factor from the cell. Thymine starvation of an F prime merozygote stimulates recombination between the episome and the chromosome. Stimulated recombination was studied in a merzygote of constitution lac-z1/F' 1ac-z2. It was found that under conditions of stimulation, the dominant lac+ recombinants and the recessive lac- homogenotic recombinants were produced with different kinetics. This suggests that recombination in an F prime merozygote is most frequently reciprocal. The kinetics of production of the lac+ segregants gave a reasonable fit to a straight line, which suggests that a recombinational event has a single rate-limiting step. An attempt to determine whether newly-formed lac+ recombinants, in the presence of an inducer, synthesised Beta-galactosidase during starvation or after reversal of inhibition, failed because Beta-galactosidase was found to be repressed during thymine starvation. It was thought possible that recombinationless mutants might possess a structural defect in their DNA, which would prevent repair and therefore recombination. To test this hypothesis, the survival after UV irradiation of lambda-bacteriophage grown on mutant and non-mutant hosts, was determined. No differences in survival were detected, and therefore, no evidence was obtained to support the above hypothesis. One of the strains used in this work was found to be sensitive to acridine orange. The genetic basis of this sensitivity was investigated, but this study was not completed.