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Index >> Regulation of Protein Synthesis - Operon >> Operator Genes

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Regulation of Protein Synthesis - Operon Structural Genes Operator Genes Promoter Gene Regulator Gene Control Mechanism - Induction Control Mechanism - Repression Inducible System Repressible System Negative Control of Protein Synthesis Positive Control of Protein Synthesis Mutation in the Operon

Operator Genes

	Operator Genes - The operator gene is adjacent to the first structural gene and controls the structural genes. It determines whether or not tbe structural genes are to be repressed by the repressor a product of the regulator gene. The operator is recognized by tbe repressor protein, which binds to tbe operator, forming an operator repressor complex. Detailed studies of the bacterial lac operator and the two operators of the bacteriophage lambda (λ) have been carried out.
The lac operator of E. coli consists of a sequence of 35 nucleotide pairs. The base pairs in the operator show a two fold symmetry (palindromic sequences), which is also found in the promoter region. The operator gene binds one active lac repressor protein, which is a tetramer of four subunits. The basic function of the operator is that on binding the repressor it physically prevents RNA polymerase from forming an initiation complex. The transcription of mRNA begins in tbe operator gene rather than at tbe polymerase binding site of the promoter region.
This is borne out by tbe fact that a complementary lac operator sequence is present at the beginning of lac mRNA. The segment of mRNA transcribed by the bases of operator gene preceeds the start codon of lac mRNA. In the lambda (λ) bacteriophage there are two operators, OL and OR (operator left and operator right). In contrast to the condition in the lac operator, the repressor binds to operators OL and OR at multiple successive sites (S1 to S and S'1 to S'.). Of these the strongest binding sites, S1 and S'1 are the ones which are nearest to the first structural gene in the operon.
One transcript starts with the tof gene which is adjacent to OR and the other with the N gene which is adjacent to OL. The repressor gene cl is located between OL and OR. The same repressor protein recognizes the two operators. OL and OR. The binding sites for the repressor on these two operators are, however not identical. OL and OR have different base sequences. Several groups of base pairs in tbe 81 site of 0L have a symmetrical arrangement, as in the lac repressor.
	It has been seen that in the lac operator there is only a partial overlapping of the promoter and operator genes. In the lambda phage, however, the promoter site is located entirely within the operator. The promoter site includes the palindromic sequence:5' G T C G A C 3' 3' C A G C T G 3'

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Regulation of Gene Activity in Eukaryotes Frenster's Model of Gene Regulation Nonhistone Protein as Derepressors Britten Davidson Operon Operator Model Sensor Genes Producer Genes Integrator Genes Receptor Genes

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