Practice question

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.