RNaselll activation of bacteriophage lambda N synthesis.

The bacteriophage lambda N gene product is one of the first genes expressed during phage development. N protein allows the expression of other phage genes by altering the transcription elongation process so as to prevent transcription termination. We have found that N levels may be modulated soon after induction or infection. Using N-lacZ fusions, we determined that cells containing RNaselll have at least a fourfold greater expression than cells defective for RNaselll. This effect is exerted at the post-transcriptional level. RNaselll processes an RNA stem structure in the N-leader RNA. Removal of the stem structure by deletion increases N expression and prevents further stimulation by RNaselll. The base of this stable stem is adjacent to the N ribosome binding site. We present a model for control of N synthesis in which this stable stem inhibits ribosome access to the N mRNA.