Structural basis of σ appropriation

Bacteriophage T4 middle promoters are activated through a process called sigma 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 sigma appropriation complex. Here, we report a single-particle cryo-electron microscopy (cryo-EM) structure of an intact sigma appropriation complex, comprising AsiA, MotA, Escherichia coli RNA polymerase (RNAP), sigma(70) and a T4 middle promoter. As expected, AsiA binds to and remodels sigma region 4 to prevent its contact with host promoters. Unexpectedly, AsiA undergoes a large conformational change, takes over the job of sigma region 4 and provides an anchor point for the upstream double-stranded DNA. Because sigma 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 sigma appropriation and transcription activation.