CORRELATION OF GENE-TRANSCRIPTION WITH THE TIME OF INITIATION OF CHROMOSOME-REPLICATION IN ESCHERICHIA-COLI

Transcriptional levels of the Escherichia coli mioC and gidA genes, wh ich flank the chromosomal origin of replication (oriC) and the dnaA ge ne, were correlated with the time of initiation of chromosome replicat ion. The transcripts were measured either in dnaC2(ts) mutants that ha d been aligned for initiation of chromosome replication by a temperatu re shift or in synchronous cultures of cells obtained using the baby m achine technique. In both types of experiments, mioC transcription was inhibited prior to initiation of chromosome replication and resumed s everal minutes after initiation. Conversely, gidA and dnaA transcripti on were both inhibited after initiation of replication, coincident wit h the period of hemimethylation of oriC DNA. It is proposed that mioC transcription prevents initiation of chromosome replication, and must terminate before replication can begin. It is further proposed that th e eclipse period between rounds of replication, i.e. the minimum inter val between successive initiations, encompasses the time required to m ethylate GATC sequences in newly replicated oriC plus the time require d to terminate mioC transcription. Conversely, the active transcriptio n of gidA and dnaA prior to initiation is consistent with their positi ve effects on initiation, and their shut-down after initiation could s erve to limit premature reinitiation.