The global regulator RNase III modulates translation repression by the transcription elongation factor N

Efficient expression of most bacteriophage lambda early genes depends upon the formation of an antiterminating transcription complex to overcome transcription terminators in the early operons, p(L) and p(R). Formation of this complex requires the phage-encoded protein N, the first gene product expressed from the p(L) operon. The N leader RNA contains, in this order: the NUTL site, an RNase III-sensitive hairpin and the N ribosome-binding site. N bound to NUTL RNA is part of both the antitermination complex and an autoregulatory complex that represses the translation of the N gene. In this study, we show that cleavage of the N leader by RNase III does not inhibit antitermination but prevents N-mediated translation repression of N gene expression. In fact, by preventing N autoregulation, RNase III activates N gene translation at least 200-fold. N-mediated translation repression is extremely sensitive to growth rate, reflecting the growth rate regulation of RNase III expression itself. Given N protein's critical role in lambda development, the level of RNase III activity therefore serves as an important sensor of physiological conditions for the bacteriophage.