DNA sequestration and transcription in the oriC region of Escherichia coli

In Escherichia coli, gidA and dnaA transcription are shut off transiently after initiation of chromosome replication, while mioC transcription is shut off before initiation. The possible involvements of seqA and dam in these transcriptional periodicities were evaluated by examining transcription of the genes in seqA and dam mutants of E. coli PC2 dnaC2(ts) aligned for initiation of chromosome replication. In both seqA- and dam- cells, gidA and dnaA continued to be transcribed after initiation, whereas the inhibition of mioC transcription before initiation was unaltered. After initiation, transcripts from mioC that traversed oriC reappeared more slowly in seqA+ dam+ cells than in seqA- or dam- cells, but before the release of oriC from sequestration. Apparently, initiation of transcription at a promoter can be completely prevented by sequestration, but established transcription forks can traverse sequestered DNA. These findings, plus analyses of transcript levels in steady-state cultures, support the idea that initiation capacity in seqA mutants is elevated because of the combined influences of increased durations of expression of both gidA and dnaA during the division cycle and defective sequestration at oriC. Accordingly, a proposal for the sequence of events during the interinitiation interval in E. coli is presented based on the evident coupling of transcription to replication.