Saccharomyces cerevisiae strain H-42 seems to have two kinds of acid phosphatase: one which is constitutive and one which is repressible by inorganic phosphate. The constitutive enzyme was significantly unstable to heat inactivation, and its Km of 9.1 x 10-4 M for p-nitrophenylphosphate was higher than that of the repressible enzyme (2.4 x 10-4 M). The constitutive and the repressible acid phosphatases are specified by the phoC gene and by the phoB, phoD, or phoE gene, respectively. Results of tetrad analysis suggested that the phoC and phoE genes are linked to the lys2 locus on chromosome II. Since both repressible acid and alkaline phosphatases were affected simultaneously in the phoR, phoD, and phoS mutants, it was concluded that these enzymes were under the same regulatory mechanism or that.they shared a common polypeptide. The phoR mutant produced acid phosphatase constitutively, and the phoR mutant allele was recessive to its wild-type counterpart. The phoS mutant showed a phenotype similar to that of a mutant defective in one of the phoB, phoD, or phoE genes. However, the results of genetic analysis of the phoS mutant clearly indicated that the phoS gene is not a structural gene for either of the repressible acid and alkaline phosphatases, but is a kind of regulatory gene. According to the proposed model, the phoS gene controls the expression of the phoR gene, and inorganic phosphate would act primarily as an inducer for the formation of the phoR product which represses phosphatase synthesis.

The phoE locus, one of the loci in which mutations lack the activity for repressible acid phosphatase, was found to be the structural gene for the enzyme by examining the enzymic characteristics of repressible acid phosphatase activity using cell extracts prepared from the leaky phoE mutants, the PHOE revertants and the PHOE recombinants between the different phoE mutants. Other evidence which strongly suggests that the phoC locus is coding for the constitutive acid phosphatase was obtained by a similar investigation. Although the phoC and phoE loci are tightly linked, they were separable by meiotic recombination.

Two new classes of mutants, phoF and phoG, lacking the constitutive acid phosphatase activity, were isolated. They both complemented each other and the phoC mutation. No linkage was detected among these three complementary genes.