<Li> Mot (swimming motility) </Li> <Li> Sm (resistance to the antibiotic streptomycin) </Li> <P> In the case of Lac, wild type cells are Lac and are able to use lactose as a carbon and energy source, while Lac mutant derivatives cannot use lactose . The same three letters are typically used (lower - case, italicized) to label the genes involved in a particular phenotype, where each different gene is additionally distinguished by an extra letter . The lac genes encoding enzymes are lacZ, lacY, and lacA . The fourth lac gene is lacI, encoding the lactose repressor--"I" stands for inducibility . </P> <P> One may distinguish between structural genes encoding enzymes, and regulatory genes encoding proteins that affect gene expression . Current usage expands the phenotypic nomenclature to apply to proteins: thus, LacZ is the protein product of the lacZ gene, β - galactosidase . Various short sequences that are not genes also affect gene expression, including the lac promoter, lac p, and the lac operator, lac o . Although it is not strictly standard usage, mutations affecting lac o are referred to as lac o, for historical reasons . </P>

How lac operon controls the metabolism of lactose