Transcription of the early region of bacteriophage T7: Characterization of the in vivo transcripts

Abstract

Twelve transcription products synthesized at early times in infection by bacteriophage T7 have been identified in ultraviolet-irradiated host cells. Their molecular weights range from 1.05 to 0.02 million and the sum is such that all twelve species can potentially fit within the early region without overlaps. The locations of the four largest transcripts were mapped by means of well characterized deletion mutations. All species appear to be metabolically stable, with their molar amounts thus reflecting the rate at which the separate sections of the early T7 DNA are transcribed. In general, the early RNAs are not present in equal molar amounts, ruling out their origin by post-transcriptional cleavage of a single precursor molecule transcribed from the entire early region. Equally important, their relative molar amounts do not fall on a 5′ to 3′ gradient corresponding to their position along the early region of T7 DNA, ruling out their origin from a single RNA polymerase binding site and successive termination and reinitiation events. Since it could be concluded that at least two different early promoters must exist, it is possible that a separate promoter exists for each different transcript. The ability to correlate a specific early phage T7 protein with a monocistronic transcript of its gene has led to the conclusion that the rate at which proteins are synthesized during phage T7 infection is mainly dependent on the efficiency of initiation sites for RNA synthesis. Although early T7 RNA is resistant to degradation, it is also known to be functionally unstable, and the existence of several low molecular weight transcripts suggests, as one possibility, that the primary transcription products have been inactivated by cleavage at or near the ribosome binding site.