It is likely that the light blue colonies have a mutation in GENE I (lacI gene). LacI (lactose inhibitor) is a DNA-binding factor that represses transcription of the lac operon.
The lacI gene (regulatory gene for lac operon) is a gene located upstream of the three genes of the lac operon (i.e., lacZ, lacY, and lacA), which are transcribed as a unit.
The inducer of the lac operon is allolactose. When lactose is present, a small amount of this molecule is converted to allolactose.
The lac repressor, which is encoded by the lacI gene, binds to the operator upstream of the lac operon, thereby preventing its transcription unless the inducer is present.
Finally, the lacZ gene produces an enzyme called β-galactosidase, while X-gal is a blue-colored product precipitated when a bacterial colony expresses a functional β-galactosidase.
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