4.2Gene Expression Regulation in Prokaryotes
The mechanism of gene expression regulation was first revealed in the β-galactosidase gene. This mechanism represents the basic functions of gene expression and suppression.
E. coli cannot directly use lactose (see Fig. 1-5 in Chapter 1), but can do so by hydrolyzing it to glucose using the β-galactosidase enzyme. E. coli bacteria cultured in a medium containing glucose do not produce the β-galactosidase enzyme (the β-galactosidase gene does not function), but if the medium contains lactose instead, they are able to use this lactose by producing the β-galactosidase protein. This mechanism, which may sound rather simple, has another characteristic; the presence of lactose does not mean the production of the β-galactosidase enzyme if glucose is also present. It is very reasonable that the β-galactosidase protein is synthesized only when lactose is present and glucose is not.
This regulation mechanism is shown in Figures. 4-1 and 4-2. The operator sequence lies in the upstream promoter region of the β-galactosidase gene. A protein constitutively produced by the lac i gene (known as a repressor) binds to the operator region, thus suppressing the action of RNA polymerase. This is the negative regulation of the β-galactosidase gene (Fig. 4-1). Under the presence of lactose, allolactose - a derivative of lactose - binds to the repressor protein and deprives it of repressor functions, thus preventing it from binding to the operator. Therefore, if RNA polymerase can bind to the promoter in the presence of lactose, mRNA can be synthesized from the β-galactosidase gene.
In lactose operon system, RNA polymerase can bind to the promoter only after the cAMP-CRP complex (i.e., a complex in which cAMP (3’, 5’-cyclic AMP) binds to CRP*3 (or CAP)) has bound to the promoter. This constitutes positive regulation of the β-galactosidase gene (Fig. 4-2). The expression of this gene is suppressed in the presence of glucose because the transport of lactose into the cell is absolutely inhibited, which disables the production of allolactose, thus keeping the repressor from being detached from the operator.
In summary, there are positive- and negative-regulation proteins in gene regulations, each of which binds to the promoter region of a gene, thus regulating its transcription. Similar regulation mechanisms can be found not only in the utilization of carbohydrates such as arabinose, but also in genes that are involved in the metabolism of amino acids and other substances.
CRP: CRP (cAMP receptor protein) is a type of transcriptional regulation factor that binds to the promoter after binding to cAMP, allowing RNA polymerase to bind to the promoter and thereby positively regulating RNA synthesis.
Mechanism of Simultaneously Regulating the Expression of Multiple Genes
E. coli bacteria have genes for enzymes that synthesize all amino acids, carbohydrates, lipids and nucleic acids from ammonia and glucose, thus regulating gene expression as necessary. As an example, when an amino acid called histidine is present in a medium, E. coli suppress all ten enzyme genes involved in histidine synthesis; when histidine is absent, E. coli simultaneously expresses these ten genes. In the case of the β-galactosidase gene, three related genes are simultaneously expressed and suppressed. These are located alongside each other in the DNA, and mRNA that reads these genes successively is synthesized. In other words, one mRNA molecule contains information on multiple genes. Such molecules are known as polycistronic mRNA (Column Fig. 4-1). The term “cistron” is synonymous with genes. An operon is a gene unit controlled by one region of gene expression regulation (i.e., one operator), and examples include lactose operons and histidine operons. Generally, in association with the synthesis and utilization of nutrients, prokaryotes have many gene regulation mechanisms that are sophisticated to fulfill their intended purposes. In this mechanism, a large number of genes form operons to generate polycistronic mRNA. Each coding region in polycistronic mRNA is bound with a ribosome, thereby synthesizing proteins. However, eukaryotes do not have operons, and therefore do not produce polycistronic mRNA.