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10.2Regulation of Gene Expression in Prokaryotes

10.2.1

Positive and Negative Regulation of the β-galactosidase Gene in E. coli

The mechanism of gene expression regulation was first revealed in the β-galactosidase gene*3. This is a fundamental mechanism that controls promotion and suppression of gene expression.
E. coli cannot directly use lactose (see Selection 4 of Chapter 6, sssFig. 6-7B); however, they can use lactose after it is hydrolyzed by their β-gal enzyme to produce glucose and galactose. E. coli cultured in a medium with glucose do not produce the β-gal enzyme (the β-gal gene does not function), but in the presence of lactose instead of glucose, they use lactose by producing the β-gal enzyme. However, the situation is not as simple as mentioned above. The β-gal enzyme is not produced when both glucose and lactose are present together at the same time in the medium. In other words, it is quite appropriate that the β-gal enzyme is synthesized only in the presence of lactose and absence of glucose.
The mechanism for this type of regulation is shown in Fig. 10-1. The operator sequence overlaps the promoter region upstream of the β-gal gene. The action of RNA polymerase is suppressed when a protein called repressor, which is constitutively expressed by i genes, binds to the operator. This is the negative regulation of the β-gal gene (Fig. 10-1A). In the presence of lactose, allolactose—the metabolite of lactose—binds to the repressor protein, which then loses its repressor function and can no longer bind to the operator. Therefore, if RNA polymerase can bind to the promoter in the presence of lactose, mRNA can be synthesized from the β-gal gene.
However, regulation is more than just that explained above. cAMP (cyclic AMP) is synthesized inside cells. In the lactose operon system, RNA polymerase can bind to the promoter only after the cAMP–CRP complex [(i.e., a complex in which cAMP binds to CRP*4 (or CAP)) has bound to the promoter. This is the positive regulation of the β-gal gene (Fig. 10-1B).
However, even in the presence of lactose, β-gal gene expression is suppressed in the presence of glucose, which is a fairly complicated mechanism. In the presence of a sufficient amount of glucose, the transporter protein that transports lactose into the cell is absolutely inhibited, and as a result, allolactose is not synthesized and the repressor does not dissociate from the operator. Thus, the β-gal gene is not expressed. This mechanism was discovered by Hiroji Aiba et al. of Japan in 1996, 40 years after François Jacob and Jacque Monod’s discovery of regulatory mechanisms.
Gene regulation involves positive and negative regulatory proteins, each of which binds to the promoter region of a gene, thus regulating its transcription. The base sequences of such regions are shown in Fig. 10-2. Similar regulation mechanisms exist not only for genes involved in the utilization of carbohydrates such as arabinose but also for those involved in the metabolism of amino acids and other substances.

*3 The β-galactosidase gene was formally referred to as the lacZ gene, but here we have referred to it as β-gal for simplicity.
*4 The prefix “epi” means “over,” “above,” or “outer.”

Fig. 10-1 Regulation of β-galactocidase gene expression

A) negative regulation by a repressor B) positive regulation by CRP

Fig. 10-2 Base sequence near expression-regulatory regions

This figure demonstrates an example of the base sequence of the region regulating β-gal gene expression. Arranged in the short region between the repressor gene and the β-gal gene, there is a region involved in the binding of multiple proteins that regulate transcription, as well as binding of RNA polymerase. The transcription-termination site of the repressor gene is not demonstrated in the figure, but it is somewhere within the region shown in this figure. In Escherichia coli, it can be found that both the genes themselves and the regions regulating thier expression are densely arranged on DNA with very little wasted space. The −10 and −35 regions are particularly important for RNA polymerase binding, and are also called core-promoters.

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Operons Regulating Simultaneous Expression of Multiple Genes

Column Fig. 10-1 Structural comparison of prokaryotic and eukaryotic mRNA

Escherichia coli possess genes for enzymes involved in the synthesis of all amino acids, carbohydrates, lipids, and nucleic acids from ammonia and glucose, and expression of the genes is regulated as required. As an example, if a culture medium contains the amino acid histidine, then genes for the 10 types of enzymes involved in histidine synthesis are all suppressed, and if the medium contains no histidine, then the genes for these 10 enzymes are all simultaneously expressed. In case of the β-gal gene, three related genes are also expressed or suppressed at the same time. These multiple genes exist in series along DNA, and mRNA that successively reads these genes is synthesized. In other words, one mRNA molecule contains information on multiple genes. This type of mRNA is called polycistronic mRNA (Column Fig. 10-1). The term “cistron” is synonymous with genes.

An operon is a functioning unit of multiple genes under the control of a single regulatory domain (operator) . For example, , lactose and histidine operons. In general, with regard to synthesis and utilization of nutrients, prokaryotes have purposive regulation mechanisms in which an operon consisting of many genes synthesizes a polycistronic mRNA. Ribosomes bind to each coding region on the polycistronic mRNA, thereby synthesizing proteins. Eukaryotes, however, do not have operons and thus do not produce polycistronic mRNA.

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Operons and Regulons

Operons are not the only method for a prokaryote to regulate simultaneous expression of multiple genes. As an example, when prokaryotes are exposed to heat shock from an increase in environmental temperature, unusual transcription initiation factors are produced. These factors induce expression of multiple genes for proteins called heat shock proteins that act against the heat shock. Genes for the heat shock proteins do not actually form a continuous operon and are rather scattered around in DNA. The system that regulates simultaneous expression of this type of scattered gene group is called a regulon. In other words, this example is a heat shock regulon. In the same manner, there is an SOS regulon that induces expression of many different DNA repair enzymes whenever DNA is damaged.

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