THE INTERACTION BETWEEN THE ASIA PROTEIN OF BACTERIOPHAGE-T4 AND THE SIGMA(70) SUBUNIT OF ESCHERICHIA-COLI RNA-POLYMERASE

The AsiA protein of bacteriophage T4 binds to the sigma 70 subunit of Escherichia coli RNA polymerase and plays a dual regulatory role durin g T4 development: (i) inhibition of host and phage early transcription , and (ii) coactivation of phage middle-mode transcription, which also requires the T4 DNA binding transcriptional activator, MotA. We repor t that the interaction between AsiA and sigma(70) occurs with a 1:1 st oichiometry, When preincubated with RNA polymerase, AsiA is a potent i nhibitor of open complex formation at the lac UV5 promoter, whereas it does not perturb preformed open or intermediate promoter complexes. D Nase I footprinting and electrophoretic mobility shift analyses of RNA polymerase-DNA complexes formed at the T4 early promoter P15.0 show t hat AsiA blocks the initial RNA polymerase binding step that leads to the formation of specific closed promoter complexes, A contrasting res ult is obtained on the T4 middle promoter PrIIB2, where AsiA stimulate s the formation of both closed complexes and open complexes, Therefore , we propose that AsiA modulates initial DNA binding by the RNA polyme rase, switching promoter usage at the level of closed complex formatio n.