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#operator mutation

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Lac Operon

lacOc mutation shows phenotype

Oc mutation alters the 21-base pair palindromic operator sequence overlapping the lac transcription start, disrupting critical base-specific contacts required for LacI repressor tetramer binding. Without stable repressor-operator interaction and tetrameric looping with auxiliary operators, promoter becomes freely accessible to sigma70 RNA polymerase holoenzyme initiating transcription. Consequently messenger synthesis of lacZYA continues even when allolactose inducer is absent. Because operator functions exclusively through cis linkage to downstream genes, the chromosome bearing Oc displays constitutive inducer-independent expression, while wild-type copy elsewhere remains regulatable, distinguishing cis-dominant constitutive from trans-recessive repressor defects.

Ref: LibreTexts 6.1.1 Use of Mutants Study lac Operon: Oc operator mutation blocks LacI binding causing constitutive.

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Lac Operon

lacOc mutation affects

Operator-constitutive mutations lacOc affect cis regulatory DNA element rather than diffusible trans-acting protein factor. Point mutations or small deletions in symmetric operator palindrome, especially central base pairs contacted by LacI recognition helix and critical guanine residues, drastically reduce LacI-operator binding affinity even for wild-type tetrameric repressor produced normally. Resulting transcription becomes constitutive but only for structural genes linked in cis to mutated operator on same DNA molecule, showing partial diploid cis-dominance in merodiploid tests, contrasting trans effects of lacI alleles. Promoter -10/-35 elements and CAP binding site remain structurally intact and functional, isolating defect to operator regulatory function alone.

Ref: Alberts Fig 7-38 lacOc mutation affects operator; NCBI - lacOc operator constitutive cannot bind repressor cis-dominant

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