Principles of Transcriptional Regulation

In Chapter 12 we saw how DNA is transcribed into RNA by the enzyme RNA polymerase. We also described the sequence elements that constitute a promoter—the region at the start of a gene where the enzyme binds and initiates transcription. In bacteria the most common form of RNA polymerase (that bearing 70) recognizes promoters formed from three elements—the “ 10”, “ 35”, and “UP” elements— and we saw that the strength of any given promoter is determined by which of these elements it possesses and how well they match optimum “consensus” sequences. In the absence of regulatory proteins, these elements determine the efficiency with which polymerase binds to the promoter and, once bound, how readily it initiates transcription. Now we turn to mechanisms that regulate expression—that is, mechanisms that increase or decrease expression of a given gene as the requirement for its product varies. There are various stages at which expression of a gene can be regulated. The most common is transcription initiation, and the bulk of this chapter focuses on the regulation of that step in bacteria. We start with an overview of general mechanisms and principles and proceed to some well-studied examples that demonstrate how the basic mechanisms are used in various combinations to control genes in specific biological contexts. We also consider mechanisms of gene regulation that operate at steps after transcription initiation, including transcriptional antitermination and the regulation of translation.