Abstract
Strains of Escherichia coli K-12 in which the transcription of lacZ is initiated from the tyrR promoter have been constructed by use of the Mu d (Apr lac) phage of Casadaban and Cohen (Proc. Natl. Acad. Sci. U.S.A. 76:4530-4533, 1979). These strains have been used to examine the regulation of expression from the tyrR promoter, with the synthesis of beta-galactosidase used as an index of expression. The specific activity of beta-galactosidase fell to 51% upon introduction of lambda (Tn10) tyrR+; to 39% upon introduction of F123, an F-prime carrying tyrR+; to 29% upon introduction of pMU309, a derivative of the plasmid RP4 carrying tyrR+; and to 13.6% upon introduction of pMU352, a derivative of the multicopy plasmid pBR322 carrying tyrR+. These results indicate that the tyrR gene product interacts with its own promoter-operator region, decreasing synthesis of beta galactosidase in the tyrR::Mu d (Apr lac) strains. The increasing extent of repression of beta-galactosidase synthesis with increasing tyrR+ gene dosage was accompanied by increasing repression of the synthesis of tyrosine- and phenylalanine-repressible 3-deoxy-D-arabinoheptulosonic acid-7-phosphate synthetases. The interaction of the repressor with tyrRo appears unusual in the sense that aporepressor alone is probably one of the repressing species. The levels of beta-galactosidase synthesized in the tyrR::Mu d (Apr lac) strains indicate that tyrR has a relatively efficient promoter, the maximum levels representing on the order of a relatively efficient promoter, the maximum levels representing on the order of 1,000 monomers of beta-galactosidase per cell in the tyrR strain and about 500 monomers in the tyrR+ haploid strain.