Identification of the N gene protein of bacteriophage lambda

The N gene protein, pN, of bacteriophage lambda stimulates early gene transcription by allowing mRNA chain elongation to proceed into genes distal to transcription termination sites normally recognized by the Escherichia coli transcription termination protein rho. pN has previously eluded detection on sodium dodecyl sulfate/polyacrylamide gels because of its small size, its instability, and the difficulty of distinguishing pN itself both from host proteins and from other early lambda proteins whose synthesis depends on pN action. These problems have now been overcome and we find that the major form of pN present in crude cell extracts of infected cells has an apparent molecular weight of 13,500. lambdabio256, a deletion-substitution mutant terminating in N, codes for a shorter pN of molecular weight 12,500. A nonsense fragment of 10,500 molecular weight coded by lambdaN(am7) has also been identified. These conclusions are based on examination of the electrophoretic profiles of the proteins synthesized after infection of UV-irradiated E. coli by various lambdaN(-) temperature-sensitive, nonsense, and deletion-substitution mutants. It has also been possible to distinguish pN itself from other early lambda polypeptides by infecting ron(-) cells with either lambdaN(mar) phage allowing pN synthesis but not pN action or lambdaN(am) phage defective in pN synthesis and pN action. Our results together with previous data are discussed with respect to the possible existence of multiple molecular weight forms of pN and the location of the coding sequences in the N gene region.