Practice question

Loss-of-function mutations designated lacI-minus, including nonsense, frameshift, or missense in DNA-binding domain, eliminate ability to produce functional repressor capable of operator binding. Lac operator remains free, RNA polymerase initiates transcription of lacZYA independently of inducer. Phenotype appears constitutive, expressed even without lactose supplementation, and recessive in trans complementation when wild-type repressor supplied from F' plasmid can restore inducibility. Such mutants were instrumental for Jacob and Monod in deducing existence of diffusible trans-acting repressor model for negative transcriptional control and operon theory foundations.