Practice question

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.