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

Efficient expression of most bacteriophage λ early genes depends upon the formation of an antiterminating transcription complex to overcome transcription terminators in the early operons, p(L) and p(R). Forma tion 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 λ development, the level of RNase III activity therefore serves as an important sensor of physiological conditions for the bacteriophage.