Genetic control of induction of alkaline phosphatase synthesis in E. coli.

Formation of the enzyme alkaline phosphatase in E. coli is under the genetic control of three genes. One of these, the P gene, specifies the structure of the enzyme molecule.1-' The other two genes, Ri and R2, regulate the rate of formation of the enzyme. In the wild-type strain (P+ Ri + R2+), the rate can vary over a thousandfold range, depending on the concentration of orthophosphate in the growth medium. The maximal rate is obtained when the concentration of orthophosphate is low; as the concentration is raised, the rate is reduced.44 The effect of a low concentration of orthophosphate on alkaline phosphatase formation is designated as induction, and that of a high concentration of orthophosphate as repression. Repression is jointly controlled by the Ri and R2 genes, as indicated by the isolation of constitutive (i.e., nonrepressible) mutants carrying mutations in either gene.7 Induction appears to be under exclusive control of the Ri gene, since mutations that cause a reduction in the rate of enzyme formation in low phosphate medium are known to occur in the Ri gene, but not in the R2 gene. Useful information about the functions of the Ri and R2 genes has been obtained from a study of heterozygous strains. With regard to repression, it was observed that if an Ri constitutive mutant was made heterozygous by the addition of a normal RI + gene, enzyme formation could be repressed.7 The same effect was found when a normal R2+ was added to an R2constitutive mutant. It was concluded from these results that repression was mediated by a cytoplasmic repressor,8 and that the production of the repressor depended upon the normal functioning of both RI and R2 genes. Induction was analyzed in an analogous way. Starting with an Ri constitutive mutant that could not produce the enzyme at the maximal rate in low-phosphate medium, it was found that this defect could be overcome by the addition of a normal Ri + gene.9 This suggests that induction is mediated by a cytoplasmic inducer specified by the Ri gene. An explanation for these findings has recently been proposed.9 It assumes that the Ri gene controls the formation of an endogenous inducer required for the synthesis of alkaline phosphatase and that the R2 gene controls the formation of an enzyme that can convert the inducer into a repressor when a high concentration of orthophosphate is present in the growth medium. This is illustrated by the following reaction scheme: Ri + gene R2+ gene I I inducer + R2 enzyme + orthophosphate repressor.