Explain why (a) inactivation of the O2 or O3 sequence of the lac operon causes o

Question

Explain why (a) inactivation of the O2 or O3 sequence of the lac operon causes only a twofold loss in repression, and (b) inactivation of both O2 and O3 reduces repression 70-fold.

The following passage wasn't included with the question, but it might be helpful: "The lac operon actually contains three operator sequences to which lac repressor binds with high affinity, known as O1, O2, and O3. O1, the primary repressor-binding site, was identified through its protection by lac repressor from nuclease digestion. The 26-bp protected sequence lies within a nearly twofold symmetrical sequence of 35 bp. O1 overlaps the transcription start site of the lacZ gene. The operator sequence O2 is centered 401 bp downstream, fully within the lacZ gene, and O3 is centered 93 bp upstream.

Explanation / Answer

Well , if you don't mind i'm giving you tye answer of both your problems together.

when we study the effect of symmetric distruction of all the three lac operators of lac operon of E.coli on repression by lac repressor.

The absence of just once pseudo operator O2 or O3 decreases repression by wild type tetrameric lac repressor approximately 2 to 3 folds .The absence of both the pseudorepressor decreases repression greater than 50 folds. O1 alone can repress at the rate of 20 folds.

The dimeric lac repressor (iadi) repressor the lac operon to about low extent, indicating that cooperative interaction between lac operon is due to DNA loop formation mediated by the lac repressor (tetrameric).Under conditions where loop formation is impossible, occupation of O3 but not O2 may lead to weak repression , suggesting that under these conditions CAP activation may be inhibited and that stops transcription at O2 which significantly do not contribute to repression.

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