Trp aporepressor production is controlled by autogenous regulation and inefficient translation.

Abstract

A trpR-lacZ gene fusion which specifies a hybrid protein that has full .beta.-galactosidase activity was constructed. The gene fusion was associated with the unaltered trpR transcription and translation control region; thus, hybrid .beta.-galactosidase production was an indicator of expression of the trp aporepressor (trpR) operon. To facilitate in vivo expression studies, a DNA segment containing the trpR-lacZ gene fusion and the trpR controlling region was transferred to phage .lambda. and subsequently inserted into the bacterial [Escherichia coli] chromosome. Analyses of hybrid .beta.-galactosidase production showed that the trpR operon is regulated autogenously but that the rate of synthesis of aporepressor varies only 4- to 5-fold in response to changes in the intracellular concentration of tryptophan. Under comparable conditions, the trp operon is regulated by trp repressor .apprx. 70-fold. Therefore, the operators of the trp operon and the trpR operon must have very different affinities for trp repressor in vivo. The promoter controlling trpR expression was moderately active. There are only .apprx. 50-300 molecules of trp aporepressor per cell. The low aporepressor level appears to be due to inefficient translation of trpR mRNA.