Mutational and functional analysis of a segment of the sigma family bacteriophage T4 late promoter recognition protein gp55.

Bacteriophage T4 late promoters, which consist of a simple 8-base pair TATA box, are recognized by the gene 55 protein (gp55), a small, highly diverged member of the sigma family proteins that replaces sigma(70) during the final phase of the T4 multiplication cycle. A 16-amino acid segment of gp55 that is proposed to be homologous to the sigma(70) region 2.2 has been subjected to alanine scanning and other mutagenesis. The corresponding proteins have been examined in vitro for binding to Escherichia coli RNA polymerase core enzyme and for the ability to generate accurately initiating basal as well as sliding clamp-activated T4 late transcription. Mutations in the amino acid 68-83 segment of gp55 generate a wide range of effects on these functions. The changes are interpreted in terms of the multiple steps of involvement of gp55, like other sigma proteins, in transcription. Effects of mutations on RNA polymerase core binding are consistent with the previously proposed homology of amino acids 68-82 of gp55 with sigma(70) region 2.2 and the recently determined structures of the Thermus thermophilus and Thermus aquaticus sigma(70)-RNA polymerase holoenzymes.