Inhibition of thymidine transport in dnaA mutants of Escherichia coli

DnaA protein is the initiator of chromosomal DNA replication in Escherichia coli. We report here our evidence that thymidine transport across cytoplasmic membranes in temperature-sensitive dnaA mutants is greatly decreased at a permissive temperature of growth of the mutants. Complementation analysis with a plasmid containing the wild type dnaA gene and P1 phage-mediated transduction confirmed that mutations in the dnaA gene were responsible for the phenotype. A low level of nucleoside transport in the dnaA mutant was specific for thymidine; transport activities for other nucleosides were much the same as those in wild type cells. Membrane vesicles prepared from the dnaA mutant showed much the same activity of thymidine transport as did those from the wild type cells. No significant difference in the activity of thymidine kinase, which is known to facilitate thymidine transport, was seen between the mutant and the wild type cells. An increase in the pool of dTTP, a negative regulator for thymidine kinase, was observed in the dnaA mutant. Based on these observations, we suggest that inhibition of thymidine transport in dnaA mutants is caused by increases in the dTTP pool.