The Operator Gene

The operator gene is adjacent to the first structural gene and controls the structural genes. It determines whether or not the structural genes are to be repressed by the repressor, a product of the regulator gene. The operator is recognized by the repressor protein which binds to the operator, forming an operator-repressor complex.

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, 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 form forming in initiation complex.

The transcription of mRNA beings in the operator gene rather than at the polymerase binding site of the promoter region. This is borne out by the 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 star 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 λ repressor bins to operators OL and OR at multiple successive sites (S1 to S6 and S’1 and S’6). 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 of OL. The repressor gene cI is located between OL and OR.

The same repressor protein recognize 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 the S1 site of OL 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 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’