Sequential Transcription of the Genes of the Lactose Operon and Its Regulation by Protein Synthesis

The initial kinetics of induction and the steady state rates of synthesis of β-galactosidase and thiogalactoside transacetylase in Escherichia coli were examined under a variety of conditions. Kinetic experiments with 5-fluorouracil and carbon starvation suggest that the z gene is transcribed into messenger ribonucleic acid (mRNA) before the a gene. In general, when protein synthesis was impaired, formation of transacetylase mRNA (defined as enzyme-forming capacity) was delayed, although β-galactosidase mRNA (similarly defined) occurred at its usual time. When protein synthesis was completely blocked, the z gene could be transcribed but the a gene could not. To explain these findings, a tentative scheme is proposed in which a gene transcription is controlled by the rate at which ribosomes travel along the completed portion of the mRNA. Since ribosomes do not appear to be as critical for the transcription of the z gene, it is suggested that the RNA polymerase maybe able to advance ahead of the leading ribosome by a certain critical distance. Kinetic experiments also suggested that the transcription of the z gene required a shorter exposure to inducer than did a gene transcription. Chloramphenicol, which specifically delayed the induction of the transacetylase, also inhibited its steady state production more than the steady state synthesis of β-galactosidase. At an antibiotic concentration of 2 µg per ml, the synthesis of β-galactosidase continued, while that of thiogalactoside transacetylase was completely inhibited.