Isolation and genetic characterization of the nitA mutants of Escherichia coli affecting the termination factor rho.

Taking advantage of the Spi (sensitivity to P2 interference) phenomenon, bacterial mutants seemingly resistant to phage lambdasusNnin5, but sensitive to phage lambdaspi, were isolated from a strain of E. coli K12 carrying no nonsense suppressor and lysogenic for P2. A class of these mutants, designated nitA (N-independent transcription), is described here. Upon infection of the nitA mutants with a trp transducing phage lambdasusN7N53ptrp46 which carries the E. coli trpE and D genes in the CIII-att region of the lambda genome, formation of anthranilate synthetase (ASase, a complex protein of trp E and D gene products) was clearly demonstrated. In contrast, no ASase formation was observed in the parent nitA+ strain under the same conditions. The synthesis is subject to "turn off" control, and is completely repressed by the CI repressor of phage lambda. The nitA cells lysogenic for lambdaCI857susN7N53 are killed by thermal induction much more efficiently than the parent cells lysogenic for the same phage. The nitA mutants support the growth of lambdasusN7N53byp much better than the parent. These results suggest that the nitA mutation permits the early leftward and rightward transcription of the lambda genome in the absence of the N gene product. On the E. coli genetic map, nitA is located between ilv and metE, nearer to ilv. The mutant allele is recessive to the wild-type allele. The present evidence, together with results of biochemical investigations to be reported, suggests that nitA is a gene specifying the transcription termination factor rho.