Molecular Genetics of Prokaryotes Problem Set

Problem 10: Phenotype of cells diploid for lac I gene

Tutorial to help answer the question

An E. coli strain that is lac I^- is conjugated with E. coli cells carrying an F' plasmid with the PI^-lac I⁺ DNA sequence on the episome. How would the expression of the lac Z⁺ be regulated in the resulting cells that are diploid and heterozygous for the lac I gene (lac I⁺ on episome and lac I^- on the bacterial chromosome)?

Tutorial

Cells diploid for lac I gene

As suggested by the diagram, the conjugated E. coli cells are diploid for the lac I gene. There is a mutant lac I^- gene on the bacterial chromosome, and a wild-type lac I⁺ gene on the F'^- DNA. The lac I⁺ gene on the F'^- DNA is expressed constitutively, leading to the production of approximately 10 lac repressor protein molecules/cell. The repressor proteins can bind to the operator DNA sequence on the bacterial chromosome, and regulate the expression of the
lac Z^- lac Y^- lac A structural genes. Thus there is normal regulation of lactose metabolism.

Notice that the creation of a partial diploid cell line, containing both lac I⁺ and lac I^- alleles can be used to demonstrate that the lac I⁺ allele is dominant.

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