Bacteriophage T4 middle promoters are activated through a process called σ appropriation， which requires the concerted effort of two T4-encoded transcription factors: AsiA and MotA. Despite extensive biochemical and genetic analyses， puzzle remains， in part， because of a lack of precise structural information for σ appropriation complex. Here， we report a single-particle cryo-electron microscopy (cryo-EM) structure of an intact σ appropriation complex， comprising AsiA， MotA， Escherichia coli RNA polymerase (RNAP)， σ70?and a T4 middle promoter. As expected， AsiA binds to and remodels σ region 4 to prevent its contact with host promoters. Unexpectedly， AsiA undergoes a large conformatio... More
Bacteriophage T4 middle promoters are activated through a process called σ appropriation， which requires the concerted effort of two T4-encoded transcription factors: AsiA and MotA. Despite extensive biochemical and genetic analyses， puzzle remains， in part， because of a lack of precise structural information for σ appropriation complex. Here， we report a single-particle cryo-electron microscopy (cryo-EM) structure of an intact σ appropriation complex， comprising AsiA， MotA， Escherichia coli RNA polymerase (RNAP)， σ70?and a T4 middle promoter. As expected， AsiA binds to and remodels σ region 4 to prevent its contact with host promoters. Unexpectedly， AsiA undergoes a large conformational change， takes over the job of σ region 4 and provides an anchor point for the upstream double-stranded DNA. Because σ region 4 is conserved among bacteria， other transcription factors may use the same strategy to alter the landscape of transcription immediately. Together， the structure provides a foundation for understanding σ appropriation and transcription activation.
