DNA replication defect in the Escherichia coli cgtA(ts) mutant arising from reduced DnaA levels

In Escherichia coli and other bacteria, the ribosome-associated CgtA GTP-binding protein plays a critical role in many basic cellular processes, including the control of DNA replication and/or segregation. However, the mechanism of this control is largely unknown. Here we report that ectopic expression of the dnaA gene partially restored both early growth in liquid medium and DNA synthesis defects of the cgtA(ts) mutant. Amounts of DnaA protein in the cgtA(ts) mutant incubated at elevated (42 degrees C) temperature were significantly lower relative to wild-type bacteria. Both level of dnaA mRNA and transcriptional activity of the dnaA promoter-lacZ fusion were decreased in the CgtA-deficient cells. The effects of ectopic expression of dnaA were specific as analogous expression of another gene coding for a replication regulator, seqA, had no significant changes in growth and DNA synthesis in the cgtA mutant. Thus, it appears that the DNA replication defect in this mutant is a consequence of reduced DnaA levels.