Interaction of T4 AsiA with its Target Sites in the RNA Polymerase σ70 Subunit Leads to Distinct and Opposite Effects on Transcription

Bacteriophage T4 AsiA is a homodimeric protein that orchestrates a switch from the host and early viral transcription to middle viral transcription by binding to the σ70 subunit of Escherichia coli RNA polymerase holoenzyme (Eσ70) and preventing promoter complex formation on most E. coli and early T4 promoters. In addition, Eσ70AsiA, but not Eσ70, is a substrate of transcription activation by T4-encoded DNA-binding protein MotA, a co-activator of transcription from middle viral promoters. The molecular determinants of σ70–AsiA interaction necessary for transcription inhibition reside in the σ70 conserved region 4.2, which recognizes the −35 promoter consensus element. The molecular determinants of σ70–AsiA interaction necessary for MotA-dependent transcription activation have not been identified. Here, we show that in the absence of σ70 region 4.2, AsiA interacts with σ70 conserved region 4.1 and activates transcription in a MotA-independent manner. Further, we show that the AsiA dimer must dissociate to interact with either region 4.2 or region 4.1 of σ70. We propose that MotA may co-activate transcription by restricting AsiA binding to σ70 region 4.1.