The nusA recognition site. Alteration in its sequence or position relative to upstream translation interferes with the action of the N antitermination function of phage lambda.

The phage lambda transcription antitermination protein, pN, acts with host factors, Nus, at sites on the phage genome, nut, to render RNA polymerase resistant to subsequent downstream termination signals. The NusA protein appears to recognize a seven to eight base-pair consensus sequence (5'Py-G-C-T-C-T-T(T)3') called boxA that is found in the promoter-proximal part of the nut region. Two types of change within or near the boxA sequence in the nutR region are shown to interfere with pN-mediated antitermination of transcription that has initiated at the upstream pR promoter. (1) A change of one base-pair (from G to T at the second position) in the boxA sequence significantly reduces pN action. (2) We prove that a frameshift mutation, cro delta 62, at the end of the gene promoter-proximal to the lambda nutR region, interferes with the pN antitermination reaction by allowing translation to proceed beyond cro into the nutR region. Using a series of plasmid constructions, we now show that the inhibition of antitermination caused by the cro delta 62 mutation can be suppressed when translation is terminated upstream from this mutation.