PhoB-dependent transcriptional activation of the iciA gene during starvation for phosphate in Escherichia coli.

The IciA protein from Escherichia coli has been shown specifically to inhibit the in vitro initiation of chromosomal DNA replication. However, the in vivo role of IciA has not yet been established. In order to investigate the in vivo function of this protein, expression of the iciA gene was studied by monitoring the beta-galactosidase activity specified by an iciA promoter-lacZ fusion inserted into the chromosome. Among the conditions tested (carbon starvation, the stringent response, phosphate starvation, and the SOS response), only phosphate depletion increased iciA expression. Supplementation of phosphate-depleted cultures with inorganic phosphate reduced the beta-galactosidase activity to basal levels. Enhanced expression of iciA-lacZ was dependent upon the PhoB protein. PhoB is known to be a transcriptional activator of the Pho regulon, expression of which is activated during phosphate starvation. It was also found that the iciA promoter contains a PhoB protein-binding sequence, termed the Pho box, which is necessary for the activation of genes of the Pho regulon. These results suggest that the iciA gene is a member of the Pho regulon.