Learning Objectives Understand the basic steps in gene regulation in prokaryotic cells Explain the roles of repressors in negative gene regulation Explain the role of activators and inducers in positive gene regulation Gene Regulation in Prokaryotes In bacteria and archaea, structural proteins with related functions—such as the genes that encode the enzymes that catalyze the many steps in a single biochemical pathway—are usually encoded together within the genome in a block called an operon and are transcribed together under the control of a single promoter.
A third genethe A gene, encodes an additional enzymetermed transacetylase, but this enzyme is not required for lactose metabolism, and we will not concentrate on it for now. This physically blocks the RNA polymerase from transcribing the tryptophan genes.
These five genes are next to each other in what is called the tryptophan trp operon Figure.
Between the promoter and the transcriptional start site is the operator region. As mentioned previously, E. The coding region includes the genes for the five tryptophan biosynthesis enzymes.
The operator mutation is dominant. A DNA sequence called the operator sequence is encoded between the promoter region and the first trp coding gene. In prokaryotes, structural genes of related function are often organized together on the genome and transcribed together under the control of a single promoter.
The high concentration of cAMP is necessary for activation of the lac operon.