The Y39A mutation of HK022 Nun disrupts a boxB interaction but preserves termination activity

Coliphage HK022 Nun protein targets phage lambda nut boxB RNA and acts as a transcriptional terminator, counteracting the phage lambda N protein, a suppressor of transcription termination. Both Nun and N protein interact directly with RNA polymerase, and Nun competes with N protein for boxB binding and prevents superinfection of Escherichia coli HK022 lysogens by lambda. Interaction of Trp18 of lambda N and A7 of boxB RNA in the N- boxB complex is essential for efficient antitermination. We found that the corresponding Nun mutation, Nun Y39A, disrupts the interaction between the aromatic ring of Y39 and A7, but the mutant retains in vivo termination activity. Stabilization of the complex by interaction of A7 with an aromatic amino acid is thus less important for Nun activity than it is for N activity. Structural investigations show similar binding of mutant and wild-type (wt) Nun protein to boxB RNA. The dissociation constants of the wt Nun(20-44)- boxB and mutant Nun(20-44)- boxB complex as well as the structures of the boxB RNA in both complexes are identical.