THE BACTERIOPHAGE-T4 ASIA PROTEIN - A MOLECULAR SWITCH FOR SIGMA 70 DEPENDENT PROMOTERS

The AsiA protein, encoded by bacteriophage T4, inhibits E sigma(70)-de pendent transcription at bacterial and early-phage promoters. We demon strate that the inhibitory action of AsiA involves interference with t he recognition of the -35 consensus promoter sequence by host RNA poly merase. In vitro experiments were performed with a C-terminally labell ed sigma factor that is competent for functional holoenzyme reconstitu tion. By protease and hydroxyl radical protein footprinting, we show t hat AsiA binds region 4.2 of sigma(70), which recognizes the -35 seque nce. Direct interference with the recognition of the promoter at this locus is supported by two parallel experiments. The stationary-phase s igma factor containing holoenzyme, which can initiate transcription at promoters devoid of a -35 region, is insensitive to AsiA inhibition. The recognition of a galP1 promoter by E sigma(70) is not affected by the presence of AsiA. Therefore, we conclude that AsiA inhibits transc ription from Escherichia coli and T4 early promoters by counteracting the recognition of region 4.2 of sigma(70) with the -35 hexamer.