The bacteriophage T4 gene mrh whose product inhibits late T4 gene expression in an Escherichia coli rpoH[formula omitted] mutant

Abstract

In an Escherichia coli rpoH mutant (affecting σ 32 , heat-shock sigma factor) infected at high temperatures with wild-type T4 phage, late T4 transcription and consequently progeny production are dramatically impaired. This defect is due, in part, to insufficient activity of σ 70 [Frazier and Mosig, J. Bacteriol. 170 (1988) 1384–1388], which is necessary to indicate early T4 transcription. Unexpectedly, however, we found that, in this rpoH host, late T4 transcription is also impaired when the temperature is raised from 30 to 42°C late after infection, when T4 transcription is directed by the T4-encoded sigma factor, σ gp55 . Here, we show that a T4 gene that we call mrh (modulates rpoH), located at 14 kb on the T4 map, is responsible for the inhibition of late T4 transcription in the rpoH mutant host. T4 deletion mutants that lack the mrh gene can produce progeny in the rpoH host, but the Mrh protein, provided in trans from a plasmid-borne mrh gene, inhibits this growth. We have cloned and sequenced this T4 gene and synthesized the Mrh protein in a T7 RNA polymerase-dependent expression system. The M r of the Mrh protein deduced from the nucleotide sequence is 13419. Gene mrh is cotranscribed with several other, yet unidentified genes, both from an early promoter downstream from the late soc gene (encoding the small outer capsid protein) and from the late soc promoter further upstream. The mrh transcript initiated at the late promoter requires antitermination downstream from soc at a palindromic sequence that could fold into a stable hairpin containing a CUUCGG loop and that terminates most, if not all, transcripts initiated from early promoters further upstream [Macdonald and Mosig, EMBO J. 3(1984) 2863–2871].