Question

A meroploid is constructed for each of the three strains. The plasmid carries a wild type lac operon and lacI gene. The production of functional B-galactoside (+ present or - absent) is measured when lactose is present and absent from the medium. What is the genotype of each of the haploid mutant strains?

Lactose Present haploid merodiploid Lactose Absent haploid merodiploid Mutant 1: Mutant 2: Mutant 3: +

Answer 1

the plasmid carries the functional lac I gene and lac operon.

mutant 1 does not express the beta-galactosidase in the presence or absence of the lactose, so the mutation may be in the beta-galactosidase gene or in the promoter if the promoter is mutated the genes in the lac operon can not be transcribed, in the merodiploid, the beta-galactosidase is expressed in the presence of lactose similar to wildtype so the genotype of the mutant 1 is lacI+ lacP+ lacO+lac Z-

or lacI+lacP-lacO+lacZ+

mutant 2, the beta-galactosidase is not expressed in any of the conditions, even in the merodiploid, so components from the lac operon in the chromosome inhibit the expression of the lac operon in the plasmid, lac Is codes for super repressor, which inhibits the expression of the lac operon by binding to the lac operator even in the presence or absence of the lactose.

the genotype of mutant 2

lac Is lacP+ lacO+ lac Z+

mutant 3 expresses lac operon in all conditions which means the lac operon in the chromosome has constitutive operon which cannot be inhibited by the lac repressor.

genotype of mutant 3.

lac I+ lac P+ lacOc lac Z+