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GENE REGULATION IN PROKARYOTES

In prokaryotic cells, only one type of RNA polymerase synthesizes all the three types of RNAs (mRNA, tRNA and mRNA) and, since, the DNA is not bounded by any nuclear membrane, transcription (formation of mRNA) and translation (formation of proteins) processes occur simultaneously in the same cytoplasm. (In eukaryotes, transcription or formation of mRNA occurs in nucleus, whereas, translation or protein synthesis in cytoplasm).

Moreover, all the proteins or enzymes needed for a metabolic pathway are generally synthesized simultaneously by the production of a single polycistronic mRNA (one cistron gives rise to

one protein), as these genes lie adjacent to each other and are transcribed continually. This polycistronic DNA along with its regulatory sites, promoter and operator, is known as an operon (Fig. 11.2). Promoter (p) is the site where RNA polymerase binds for starting the transcription, whereas, operator (o) is a small part of promoter that is recognized and bound by the regulatory” protein synthesized by a regulator gene (i), which is not a part of the operon and lies at a distance (up- stream) from the operon. Depending upon the type of metabolic activity, the regulation of gene expression can be of inducible type or of the repressible type. The synthesis of enzymes required for most of the catabolic pathways, e.g., utilization of lactose sugar, are the inducible systems, where the presence of lactose, for example, induces the expression of necessary genes. On the other hand, the presence of a metabolite, e.g., tryptophan amino acid supplemented in nutrient medium, may inhibit the synthesis of all the enzymes necessary for its production (anabolic pathways). Such systems are known as repressible systems,

Jacob and Monod (1961) were the first to give the models for explaining the induction or repression of enzyme synthesis in E. coli. 

Fig.11.2

Fig. 11.2 An operon.

 The protein synthesis can be regulated either at the transcriptional level or at the translational level. Some of the well-studied operons in E. coli are:

 1. Lac Operon

In E. coli, the lac operon, which is an inducible system, consists of promoter, a operator and three structural genes. These structural genes, after transcription and translation.

give rise to the three enzymes, β-galactosidasc (product of z structural gene), β-galactosid permease (product of y structural gone) and β-galactoside transacetylase (product of a

structurul gene), which are necessary for the metabolism (catabolism) of lactose sugar (a
β-galactoside) if present in external nutrient medium (minimal nutrient medium, that is required for the growth of organism, supplemented with lactose) (Fig.11.3)

β-galactosidase catalyses the reaction

Lactose → Glucose + Galactose

Β-galactoside permease helps in the transport of lactose into the cells from external medium.

Fig.11.3

Fig. 11.3 Structure of lac operon in E. coli.

 β-galactoside transacetylase transfers the acetyl group from acetyl CoA to β-galactoside sugars. It also helps in the utilization of non-metabolizable analogues of β-galactoside, which results in the detoxification and excretion of these analogues

 i) When lactose is absent in nutrient medium. The regulator gene i synthesizes a repressor protein that recognizes and binds at the operator site in the absence of lactose in external medium and, thus, inhibits the synthesis of polycistronic mRNA by checking the movement of RNA polymerase at the promoter site (Fig. 11.4 and 11.6).

Fig.11.4

No mRNA

Fig. 11.4 Lac-operon in E.coli when lactose is absent in external medium

 When lactose is present in nutrient medium. When lactose is added in the minimal nutrient medium, the lactose binds with the repressor making it inactive so that it becomes unable to attach at the operator site. This leads to the synthesis of mRNA and, after, translation, the three enzymes that are used in the metabolism of lactose. Thus, lactose acts as the inducer of gene expression (Fig. 11.5 and 11.6). [After the repressed state, when lactose is added in the external medium, it manages to enter the cell with the help of β-galactoside permease, as a minimal amount of this enzyme is always present to start the metabolic pathway]

[Actually, lac operon normally remains in repressed state and whenever lactose is added in minimal nutrient medium, it becomes induced.]

Instead of lactose, allolactose that is the metabolic product of galactose, is the actual inducer.

Lactose → Glucose + Galactose

Allolactose + Galactobiose]

Fig.11.5

Fig. 11.5 Functioning of lac operon in the presence of lactose in E.coli.

 

2. Tryptophan Operon

In E. coli, the trp operon, which is a repressible system, consists of promoter, operator and five structural genes, trpE, trp D, trp C, trp R and trp A, that after transcription and translation give rise to ·the respective enzymes necessary for the synthesis of tryptophan amino acid, if it is not present in external nutrient medium.

(i)  When tryptophan is present In nutrient medium, The regulatoor gene I synthesizes an inactive repressor protein that after binding with the compressor (tryptophan) present in external medium recognizes and binds at the operator site, thus, inhibits the synthesis of polycistronic mRNA by checking the movement of RNA polymerase at the promoter site (Fig.11.7).

Fig.11.6

Fig. 11.6 Lac-operon in E.coli.

 (ii) When tryptophan is absent in nutrient medium. When tryptophan is absent in the external medium, the inactive repressor is unable to bind at the operator site, thus, leading to the synthesis of mRNA and the corresponding enzymes that are used in the synthesis of tryptophan (Fig. 11.8).

[Normally, the trp operon remains in expressed form, and whenever tryptophan is added in the minimal nutrient medium, it is repressed.]

Fig.11.7

Fig. 11.7 Trp operon when tryptophan is present in external medium.

Fig.11.7                                                                                          inactive

Fig.11.8

Fig.11.8 Trp operon, when tryptophan is absent in external medium, in E coli

Post-transcriptional Regulation of Tryptophan Operon

(i) When tryptophan is present in nutrient medium. One more type of control system of the trp operon occurs post-transcriptionally (after the start of transcription process) that is based on the synthesis of a leader polypeptide when tryptophan is present in the external medium. A 162 base pair long DNA found at the 5′ end of the gene trp E (part or gene E), having two successive tryptophan codons, is responsible for the synthesis of leader polypeptide. This regulation is based on the coupling of transcription and translation processes in prokaryotes. After the initiation of transcription, ribosomes one by one get attached to the 5′ end of the newly synthesizing mRNA and start the synthesis of the respective proteins. In the presence of tryptophan, the two tryptophan codons are translated due to the availability of tryptophan in external medium, thus, forming the complete leader polypeptide. This leader polypeptide induces the formation of hairpin loop in the newly synthesizing mRNA, causing the detachment of RNA polymerase from the coding strand of DNA. Thus, termination of further transcription occur (Fig. 11.9).

Fig.11.9

Fig. 11.10 Post-transcriptional control of trp operon in the absence of tryptophan.