ESCHERICHIA-COLI SALMONELLA-TYPHIMURIUM HYBRID NUSA GENES - IDENTIFICATION OF A SHORT MOTIF REQUIRED FOR ACTION OF THE LAMBDA-N-TRANSCRIPTION ANTITERMINATION PROTEIN

The Escherichia coli nusA gene, nusA(Ec), encodes an essential protein that influences transcription elongation. Derivatives of E. coli in which the Salmonella typhimurium nusA gene, nusA(St), has replaced nusA(Ec) are viable. Thus, NusA(St) can substitute for NusA(Ec) in supporting essential bacterial activities. However, hybrid E. coli strains with the nusA(St) substitution do not effectively support transcription antitermination mediated by the N gene product of phage lambda. We report the DNA sequence of nusA(St), showing that the derived amino acid sequence is 95% identical to the derived amino acid sequence of nusA(Ec). The alignment of the amino acid sequences reveals scattered single amino acid differences and one region of significant heterogeneity. In this region, called 449, NusA(Ec) has four amino acids and NusA(St) has nine amino acids. Functional studies of hybrid nusA genes, constructed from nusA(Ec) and nusA(St), show that the 449 region of the NusA(Ec) protein is important for lambda N-mediated transcription antitermination. A hybrid that has a substitution of the four E. coli codons for the nine S. typhimurium codons, but is otherwise nusA(St), supports the action of the N antitermination protein. The 449 region and, presumably, adjacent sequences appear to compose a functional domain of NusA(Ec) important for the action of the N transcription antitermination protein of phage lambda.