Molecular Genetics of Prokaryotes Problem Set

Problem 3: Phenotype of lac Z E. coli cells

Tutorial to help answer the question

An E. coli strain is lac Z. The structural gene for ß-galactosidase is encoded at the lac Z locus. How would you describe the regulation of lactose metabolism in these cells?


Phenotype of a lac Z cell

The term lac Z (read as "lac Z minus,") means the DNA that encodes ß-galactosidase, the E. coli lac Z gene product, is defective or absent due to mutation. The phenotype of a cell that is lac Z is an inability to synthesize the lac Z gene product, ß-galactosidase. Without this enzyme, cells cannot metabolize lactose, and fail to grow if lactose is the only energy source.

There are several types of mutation that can cause a lac Z phenotype. These include:

Nonsense mutation
A codon for an amino acid is mutated to a termination codon (UAA, UAG, UGA), causing premature termination during protein synthesis.
Missense mutation
A codon for an amino acid is mutated to a codon for a different amino acid. The new amino acid substitution renders the enzyme inactive.
Deletion mutation
A portion of the DNA of the lac Z gene is absent. With enzyme coding information absent, a functional enzyme cannot be synthesized.

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Updated: July 15, 1999
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