REGULATION OF ESCHERICHIA-COLI TOPA GENE-TRANSCRIPTION - INVOLVEMENT OF A SIGMA(S)-DEPENDENT PROMOTER

To investigate the regulation of Escherichia coli topA gene transcript ion, primer extension was employed to determine the transcription init iation sites from the chromosomal topA gene. When cells were grown in LB medium to log phase, four transcription initiation sites could be i dentified. Three of these sites corresponded to promoters P1, P2 and P 4 previously characterized using topA-galK fusion plasmids. The P3 pro moter that is active on the plasmid was not utilized at the chromosoma l topA gene under the conditions employed. There was a new transcripti on initiation site corresponding to a new promoter Px1. When cells sta rted to enter stationary phase, promoter Px1 gradually became the majo r transcription initiation site for topA, while transcription from pro moters P2 and P4 decreased. In an E. coli mutant lacking sigma(s) (the rpoS gene product), the stationary phase specific sigma factor, the i nduction of transcription from promoter Px1 was abolished. In another mutant lacking H-NS activity, resulting in increased sigma(s) level in log-phase, the transcription from promoter Px1 during log phase was i ncreased. Thus Px1 appeared to be regulated by sigma(s). Thee activity of promoter P1 on the chromosome increased during heat shock, consist ent with the previous result obtained using the topA-galK fusion plasm id showing that P1 is a sigma(32)-dependent heat shock promoter. Promo ters P2 and P4 were most likely to be recognized by sigma(70). The tot al level of topoisomerase I protein in the rpoS mutant was not reduced significantly in stationary phase due to increased transcription init iation from the other topA promoters. The utilization of multiple sigm a factors for transcription initiation of topA could be important for adaptation of E. coli to change in growth conditions. (C) 1997 Academi c Press Limited.