Inhibition of Escherichia coli RNA polymerase by bacteriophage T4 AsiA

The 10 kDa bacteriophage T4 antisigma protein AsiA binds the Escherichia coli RNA polymerase promoter specificity subunit, sigma(70), with high affinity and inhibits its transcription activity. AsiA binds to sigma(70) primarily through an interaction with sigma(70) conserved region 4.2, which has also been implicated in sequence-specific recognition of the -35 consensus promoter element. Here we show that AsiA forms a stable ternary complex with core RNA polymerase (RNAP) and sigma(70) and thus does not inhibit sigma(70) activity by preventing its binding to core RNAP. We investigated the effect of AsiA on open promoter complex formation and abortive initiation at two -10/-35 type promoters and two "extended -10" promoters. Our results indicate that the binding of AsiA to sigma(70) and the interaction of sigma(70) region 4.2 with the -35 consensus promoter element of -10/-35 promoters is mutually exclusive. In contrast, AsiA has much less effect on open promoter complex formation and abortive initiation from extended -10 promoters, which lack a -35 consensus element and do not require sigma(70) conserved region 4.2. From these results we conclude that T4 AsiA inhibits E. coli RNAP sigma(70) holoenzyme transcription at -10/-35 promoters by interfering with the required interaction between sigma(70) conserved region 4.2 and the -35 consensus promoter element. (C) 1998 Academic Press Limited.