Stringent control in Escherichia coli applies also to transcription by T7 RNA polymerase

During amino acid starvation the synthesis of rRNA and tRNA is specifically inhibited (stringently controlled) in wild type Escherichia coli but not in relaxed strains carrying the relA mutation. We have found that the in vivo transcription of a hybrid rrnB rRNA operon, in which the normal promoter region has been replaced by the lambda PL promoter, is under stringent control even though this promoter lacks the "stringent discriminator" sequence postulated to be required for stringent control. Furthermore, we have found that transcription of the rrnB operon from a phage T7 promoter, as well as T7 genes in general, by phage T7 RNA polymerase is also subject to stringent control in vivo. These results are consistent with the idea that stringent control acts in a relatively nonspecific manner to inhibit some step(s) in transcription that are often rate-limiting for very active transcription. The relative simplicity of transcription by phage T7 RNA polymerase should offer a good system to study the molecular mechanisms of stringent control.