Specificity of the bacteriophage lambda N gene product (pN): nut sequences are necessary and sufficient for antitermination by pN.

We have cloned the nutR site together with the tR1 site of bacteriophage lambda in the E. coli galactose operon to examine whether the lambda promoter sequences PR and PL are involved in the recognition specificity of the lambda N gene product (pN). We first constructed a derivative of plasmid pBR322 in which the expression of the tetracycline genes (tet) is controlled by the gal promoter (Pgal). This new plasmid contains a unique Hind III site between Pgal and tet into which the nutR and tR1 sites were introduced. The order of the relevant genetic markers in this second plasmid is Pgal-nutR-tR1-tet. Cells transformed with this plasmid express tet only if pN is provided and if the plasmid contains an intact gal promoter. Our data suggest that transcription which originates at Pgal is modified by pN at nutR, enabling it to pass through tR1 into tet. We conclude that promoters do not play a specific role in pN recognition and that nut sequences are both necessary and sufficient for pN action.