Phage HK022 Nun protein represses translation of phage λ N (transcription termination/translation repression)

Abstract

The N-terminal arginine-rich motif of phage HK022 Nun protein binds to NUT sequences in phage nascent transcripts and induces transcription termination. Interactions between the Nun C terminus and RNA polymerase as well as the DNA template are required for termination. We have isolated Nun C-terminal point and deletion mutants that are unable to block transcription. The mutants bind NUT RNA and inhibit translation of the N gene. Thus HK022 excludes both by terminating transcription on the phage chromosome and by preventing translation of the essential N gene. Like N autoregulation, translation repression by Nun requires host RNaseIII deficiency (rnc) or a mutation in the RNaseIII processing site (rIII) located between NUTL and the beginning of the N coding sequence. Our data support the idea that Nun bound at NUTL causes steric interference with ribosome attachment to the nearby N coding sequence. Two models, Nun acting alone or in complex with host proteins, are discussed.

Revisited gene regulation in bacteriophage λ

“…N-dependant repression can be abolished by disruption of any of the other components of the antitermination complex (for example, by mutations of nutL or Nus proteins or by replacing E. coli RNAP, itself, with T7 RNAP, which does not interact with N) [28 ,29]. However, repression is retained if N is substituted by the HK022 Nun protein, which, like N, binds to nutL and RNAP [30]. The RNA stem loop juxtaposing nutL and N-RBS can be cleaved by the host RNaseIII, cutting the tether between N and N-RBS.…”

Translation repression by an RNA polymerase elongation complex

“…Here, a completely functional interaction between Nun and RNAP, i.e. one that causes termination, is not required for translation repression (Kim et al ., 2003).…”

RNA Polymerase Elongation Factors

“…Nun binding to Nut sites causes termination, not antitermination. Furthermore, Nun binding blocks translation of N protein mRNA (52). The HK022 lysogen constitutively expresses Nun, which arrests transcribing RNAP and destroys the transcription program of a sensitive phage (54,112), no doubt acting to prevent superinfection by competing phages (20,96,112,115).…”