DNA synthesis in an Escherichia coli dna B dnaC mutant

An Escherichia coli K12 dnaB dnaC mutant was constructed by P1 transduction of the dnaC allele into a dnaB recipient stain. dnaB dnaC transductant were discriminated from dnaB mutants by their inability to grow at 40 degree C after lysogenization with phage P1bac. The dnaB dnaC mutant character was verified by 1. P1 transduction, and 2. by in vitro complementation with dnaB and dnaC wild type protein fractions. DNA synthesis was studied in strains containing dnaB, dnaB dnaC alleles in an otherwise uniform genetic background with the dnaB character either unsuppressed or suppressed by P1bac prophage. Degradation at 42 degree C of [3H]-thymidine pulse-labeled DNA in dnaB and dnaB dnaC mutants is suppressed by P1bac. However, unlike the dnaC mutant, the P1bac lysogen of the dnaB dnaC mutant exhibits an abrupt cessation of DNA synthesis and less residual cell divisions at 42 degree C indicating an inhibition of DNA chain elongation rather than a defect in DNA initiation. It is suggested that denaturation of the dnaB protein effects the dnaC function.