One colony was discovered to have changed from inducible to constitutive lac expression. What is the most likely cause?

While today site-directed mutagenesis has greatly streamlined research in genetic expression, labs that are less well funded or reliant on older techniques can still produce advances within the field, albeit at a much slower pace. One such researcher was utilizing near-visible ultraviolet light as a mutagen in investigating the efficiency of such light sources used in commercial bakeries. This researcher was utilizing Serratia marcescens, a coliform bacterium, as the reporter organism because of its intense red pigment when grown under standard conditions. The technique being used is... Show more

While today site-directed mutagenesis has greatly streamlined research in genetic expression, labs that are less well funded or reliant on older techniques can still produce advances within the field, albeit at a much slower pace. One such researcher was utilizing near-visible ultraviolet light as a mutagen in investigating the efficiency of such light sources used in commercial bakeries. This researcher was utilizing Serratia marcescens, a coliform bacterium, as the reporter organism because of its intense red pigment when grown under standard conditions.
The technique being used is replica plating, where organisms are irradiated and grown on nutritionally complete agar medium, and then the respective colonies are transferred via sterile cloth-covered plates that reproduce the exact same colony location onto media that are deficient in one or more components. Mutants are detected by their colonial presence on the complete medium but their absence on the deficient medium. The colony present on the original medium is then used as the source for mutant studies. In this case, the lab is studying lac operon expression. Show less