By the end of this section, you will be able to:
- Describe the steps involved in prokaryotic gene regulation
- Explain the roles of activators, inducers, and repressors in gene regulation
The DNA of bacteria and archaea is usually (there are a few known exceptions to the circular chromosome in bacteria) organized into a circular chromosome supercoiled in the nucleoid region of the cell cytoplasm. Proteins that are needed for a specific function, or that are involved in the same biochemical pathway, are often times encoded together in blocks called
operons . Therefore, operons are single transcription units, encoding for multiple genes. Expression of these genes is organized from a single regulatory region and all genes in the operon are therefore regulated as a single unit. For example, all of the genes needed to use lactose as an energy source are coded next to each other in the lactose (or
lac ) operon.
In bacteria, all transcription is controlled through RNA polymerase, a multiprotein complex that recognizes the promoter region and initiates transcription, elongates the transcript, and terminates transcription. Therefore, gene expression can be regulated at any of these steps, initiation, elongation, or termination; however, in bacteria, the majority of the regulation is at the level of transcription initiation.
The first level of control of gene expression is at the promoter itself. There are two ways a promoter controls gene expression. First is which RNA polymerase holoenzyme (sigma + Core RNA polymerase) recognizes the promoter. Remember, bacteria have a number of sigma factors many of which control gene expression only under certain conditions, such as Sigma-S during stationary phase.
The second level of control is promoter strenght, some promoters are considered "strong", while others are considered "weak". The basis of promoter strength is the specificity the promoter has to RNA polymerase. Each different sigma factor has a different recognition sequence, for example, the sigm-70 protein in
E. coli has the recognition sequence 5'-TTGACA-(16-17 nucleotides)-TATAAT-3'. Strong promoters have sequences close to the consensus recognition sequence, weak promoters have sequences more divergent to the consensus.
Regulator proteins
The next layer of control is the addition of
regulatory proteins. These proteins can either act to increase transcription, and are often called
activators or
activator proteins . These proteins bind to the promoter region and aid RNA polymerase to recognize a promoter and initiate transcription. Alternatively, regulatory proteins that inhibit transcription are often referred to as
repressors or
repressor proteins . Some regulatory proteins can act both as a repressor or an activator depending upon how they interact with RNA polymerase and the promoter. For example the regulatory protein called CAP can act to activate some genes and repress other genes. Therefore the terms "activator" and "repressor" should be used depending upon the situation or condition, and may not truely reflect the role of the protein in question.
