DNA SEQUESTRATION AND TRANSCRIPTION IN THE ORIC REGION OF ESCHERICHIA-COLI

In Escherichia cell, gidA and dnaA transcription are shut off transien tly after initiation of chromosome replication, while mioC transcripti on is shut off before initiation. The possible involvements of seqA an d dam in these transcriptional periodicities were evaluated by examini ng transcription of the genes in seqA and dam mutants of E. cell PC2 d naC2(ts) aligned for initiation of chromosome replication. In both seq A(-) and dam(-) cells, gidA and dnaA continued to be transcribed after initiation, whereas the inhibition of mioC transcription before initi ation was unaltered. After initiation, transcripts from mioC that trav ersed 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 complet ely prevented by sequestration, but established transcription forks ca n traverse sequestered DNA. These findings, plus analyses of transcrip t levels in steady-state cultures, support the idea that initiation ca pacity 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 pro posal for the sequence of events during the interinitiation interval i n E. cell is presented based on the evident coupling of transcription to replication.