Sigma competition: The contest between bacteriophage T4 middle and late transcription

In bacterial transcription, diverse sigma-family promoter recognition prote ins compete for a common RNA polymerase core. Bacteriophage T4 infection ul timately reduces this competition to a duel between activated viral middle and enhanced late transcription, involving two sigma proteins, two phage-en coded activator proteins and two phage-specific co-activators. This competi tion has been analyzed in vitro, and the relative abundances in T4-infected Escherichia coli of the participating proteins have been measured. Activat ed late transcription holds an advantage over activated middle transcriptio n, especially at higher ionic strength. This advantage is further compounde d by ADP-ribosylation of the RNA polymerase alpha subunits, and by the phag e-specific, RNA polymerase core-bound RpbA subunit. The largest contributio n to the middle-late competition is made by gp55, the late sigma factor, bu t not enough of gp55 is produced during T4 infection to shut off middle tra nscription by direct competition with sigma 70. AsiA, the originally identi fied anti-sigma protein is not a major determinant of middle-late competiti on. (C) 1999 Academic Press.