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:
A codon for an amino acid is mutated to a termination codon (UAA, UAG, UGA), causing premature termination during protein synthesis.
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.
A portion of the DNA of the lac Z gene is absent. With enzyme coding information absent, a functional enzyme cannot be synthesized.