In vivo role of the relA+ gene in regulation of the lac operon.

Under conditions of amino acid limitation, beta-galactosidase was produced at a 70-fold higher rate in a relA+ strain than in an isogenic relA strain of Escherichia coli K-12. Under identical conditions with the relA+ and relA strains carrying various lac promoter mutations, rates of beta-galactosidase synthesis in relA+ (relative to relA) ranged from 26-fold higher (promoter mutant Pr 13) to only 5-fold higher (promoter mutant PrL8uv5). This promoter specificity was independent of strain background and the means of eliciting amino acid limitation. Addition of cyclic AMP to the growth medium altered the relA+/relA difference for beta-galactosidase synthesis from the wild-type lac promoter. The experiments suggest that the relA+/relA difference in lac expression arises primarily at the point of transcription initiation. The results are discussed in relation to recent in vitro data showing a promoter-specific guanosine 5'-diphosphate 3'-diphosphate stimulation of lac transcription (P. Primakoff and S. W. Artz, Proc. Natl. Acad. Sci. U.S.A. 76:1726-1730).