An RNA polymerase alpha subunit mutant impairs N-dependent transcriptional antitermination in Escherichia coli

We show that the rpoA341 mutation in the gene encoding the alpha subunit of Escherichia coli RNA polymerase results in a decreased level of transcripts originating from the lytic promoters PL and PR of infecting lambda phage. However, using lacZ fusions we demonstrate that initiation of transcription from both PL and PR is not impaired in the rpoA341 host. Rather, it is the level of the longer, antiterminated PL- and PR-derived transcripts which is altered: the activity of beta-galactosidase in bacteria harbouring a source of N and a PL-nutL-tL1-tI-lacZ or PR-nutR-tR1-lacZ fusion is considerably lower in the rpoA341 mutant relative to the rpoA+ strain. In the absence of the antiterminator protein N no difference is observed in the level of longer PR-derived transcripts between wild-type (rpoA+) and mutant (rpoA341) hosts. Although synthesis of N appears to be similar in both phage-infected rpoA+ and rpoA341 cells, overexpression of the N gene leads to restoration of wild-type levels of the longer PL- and PR-derived transcripts in the mutant host. While this mutation does not appear to affect vegetative phage growth in nus+ backgrounds, in combination with certain nus mutations it retards lytic development. Therefore, we conclude that the rpoA341 mutation specifically interferes with the function of the N-antitermination complex, suggesting that the C-terminal domain of the RNA polymerase alpha subunit may play an important role in N-dependent transcriptional antitermination.