Identification of a potential control region in bacteriophage T7 late promoters

Abstract

During infection of Escherichia coli, bacteriophage T7 encodes a simple RNA polymerase that consists of a single protein species of 883 amino acids^1,2. This phage-specified RNA polymerase transcribes the late genes of T7 in two temporal classes (class II and class III)³. The mechanism by which late transcription is regulated is unclear. In general, class II promoters are weaker than class III promoters and in vitro the purified phage RNA polymerase discriminates between these classes of promoters as a function of ionic conditions and agents that affect the stability of the DNA helix⁴. Sequence analysis of 17 promoters utilized by the phage RNA polymerase reveals a highly conserved 23 base-pair (bp) consensus sequence that is different from known bacterial promoters⁵. To determine which structural features of the late promoters are important for the regulation of transcription, we have constructed a number of class II/III hybrid promoters and have determined their properties both in vitro and in vivo. The results reported here indicate that an A + T-rich region that overlaps the upstream end of the promoter has an important role in the control of promoter selection during T7 infection. In addition, changes in the 23-bp conserved promoter sequence may also affect promoter regulation.