Operon Hypothesis

Regulation of gene action or Genetic control of gene action
Regulation of Gene Action or Genetic Control of Gene Action
Enzyme Regulation of Gene Action
Operon Hypothesis
Genetic Properties of Bacterial Operons
Promotor Gene
Operator Gene
Regulatory Gene and Regulator Protein
Inducer or Effector Molecule
Regulation of the Lac Operon of Ecoli
Molecular Biology of Lac Operon
Catabolite Repression
Control of Gene Expression in Eukaryotes

Operon Hypothesis

	Operon Hypothesis According to recent version of operon hypothesis, the genetic regulation in bacteria and bacteriophages is performed by following five key elements: 1.operon or regulated structural genes; 2. the promoter; 3. the operator; 4. the regulatory gene and regulator protein; 5. the effector or inducer. 1. Operon or Regulated Structural Genes In bacteria and bacteriophages different genes concerned with the same trait are often found clustered together in a group known as an operon. For example, the genes in a bacterial operon determine structure of those enzymatic proteins which control sequential metabolic steps of a biochemical reaction. Examples of Bacterial Operons In E. coli and Salmonella typhimurium, a variety of operons have been investigated. Few important operons of E. coli can be considered here as examples:
(i) Gal operon of E. coli-In the gal operon of E. coli, the sequence of genes (e.g., gal E, gal T, and gal k) in the operon directly mirrors the sequence followed by the gene products (e.g., galactokinase, galactose-I-phosphate uridyl transferase, and uridinediphosspho-galactose-4-epimerase) as they carry out the degradation of galactose into glucose-I-phosphate and UDP-glucose. (ii) Lac operon or E. coli: - The lac operon of E. coli includes three structural genes (z. y, and a) and among its three structural genes, only the z gene specifies an enzyme) mown to be direct involved in the metabolic pathway for lactose utilization. The z gene product in a single long polypeptide chain (134,000 daltons); this combines with three other like chains to form the tetrameric enzyme β-galactosidase which catalyzes the hydrolysis of lactose (or other β-galactosidaes) into galactose and glucose.
The y gene product, on the other hand does not specify a catabolic enzyme at all but rather a molecule known as M protein or galactoside permease which is located in the bacterial cell membrane and acts to facilitate the specific uptake of lactose (and similar molecules) from the external medium. The last gene in the operon, gene a, specifies enzyme thiogalactoside transacetylase which can, in vitro, transfer the acetyl group from acetyl coenzyme A to isopropyl-D-thiogalactoside (IPTG) a synthetic galactoside derivative.The function of this enzyme in vivo is unknown, but since bacterial strains carrying deletions of the a gene behave normally in all respects, the gene appears not to specify an enzyme essential for lactose utilization.

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