The yeast Saccharomyces cerevisiae has at least six species of acid and alkaline phosphatases with different cellular localizations, as well as inorganic phosphate (P i ) transporters. Most of the genes encoding these enzymes are coordinately repressed and derepressed depending on the P i concentration in the growth medium. The P i signals are conveyed to these genes through a regulatory circuit consisting of a set of positive and negative regulatory proteins. This phosphatase system is interested as one of the best systems for studying gene regulation in S. cerevisiae due to the simplicity of phenotype determination in genetic analysis. With this methodological advantage, considerable amounts of genetic and molecular evidence in phosphatase regulation have been accumulated in the past twenty-five years. This article summarizes the current progress of research into this subject.

Mutants of Saccharomyces cerevisiae that require cAMP for growth have been isolated. Some of the mutants isolated were deficient in adenylate cyclase activity and mapped at a locus, cyrl, located near the centromere of chromosome X.Growth of cells carrying the cyrl mutation was arrested at the GI phase of the yeast cell cycle in the absence of cAMP. The cyrl mutation was suppressed by a secondary mutation designated bcyl. The bcyl mutation bypassed the need for cAMP for growth. The bcyl mutants had extremely low levels of cAMP-binding protein and cAMP-dependent protein kinase but produced a high level of cAMP-independent protein kinase. The results indicate that cAMP is an essential factor for yeast cells to proceed through the cell cycle via the activation of protein kinase.

Seven dominant mutations showing greatly enhanced resistance to the glucose repression of galactokinase synthesis have been isolated from GAL81 mutants, which have the constitutive phenotype but are still strongly repressible by glucose for the synthesis of the Leloir enzymes. These glucose-resistant mutants were due to semidominant mutations at either of two loci, GAL82 and GAL83. Both loci are unlinked to the GAL81-gal4, gal80, or gal7-gallO.-gall locus or to each other. The GAL83 locus was mapped on chromosome V at a site between arg9 and chol. The GAL82 and GAL83 mutations produced partial resistance of galactokinase to glucose repression only when one or both of these mutations were combined with a GAL81 or a gal80 mutation. The GAL82 and GAL83 mutations are probably specific for expression of the Leloir pathway and related enzymes, because they do not affect the synthesis of a-D-glucosidase, invertase, or isocitrate lyase.In the on-off control of the lac operon in Escherichia coli, there is dual control of enzyme synthesis (9): negative control by a repressor coded by the lacI gene, and positive control by a complex molecule consisting of a specific catabolite gene activator protein (CAP protein) and cyclic adenosine 3',5'-monophosphate. The repressor detects the presence or absence of,-galactoside in the cytoplasm, and the CAP protein detects glucose through the alteration of the intracellular level of cyclic adenosine 3',5'-monophosphate. These signals are conveyed to the appropriate sites in the promoter region of the lac operon.Glucose repression or carbon catabolite repression is also commonly observed in yeasts.Recent biochemical studies clearly demonstrate that glucose repression of cytochrome c synthesis in Saccharomyces cerevisiae is at the level of gene transcription (32, 33), whereas the contribution of cyclic adenosine 3',5'-monophosphate is, in general, ambiguous in yeasts. To elucidate the genetic mechanism of carbon catabolite repression in yeast, mutants with altered regulatory properties have been isolated and characterized by several workers. These include mutations producing resistance to carbon catabolite repression in invertase synthesis (15, 24), pleiotropic mutations conferring resistance to carbon catabolite repression (5, 6), the hex mutation (13) with reduced activity of glucose phosphorylation, which contributes to carbon catabolite repression, and the ADR mutations (4, 7) in a regulatory system for glucose repression of alcohol dehydrogenase synthesis. The genetic data, however, are still insufficient to construct a genetic model for carbon catabolite repression.In previous communications (22, 23), we proposed a genetic model for the role of the inducer in the synthesis of the galactose pathway enzymes (the Leloir enzymes) in S. cerevisiae. The structural genes for the Leloir enzymes, the gall locus (encodes galactokinase [EC 2.7.1.6]), gal7 (encodes a-D-galactose-1-phosphate uridylyltransferase [EC 2.7.7.12]), and gallO (encodes uridine diphosphoglucose 4-epimerase [EC 5.1.3....

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.

Two dominant uninducible mutant alleles in the gal80 locus were identified. The GAL80s-1 and GAL80s-2 mutants showed novel phenotypes in response to the newly isolated GAL81-1 mutant allele, a dominant constitutive mutation linked to the gal4 locus; the GAL80s-1 GAL81-1 strain was inducible and the GAL80s-2 GAL81-1 strain was uninducible. Many galactose positive revertants from the GAL80s-2 GAL81-1 strain were isolated. It was proved that each revertant was due to a secondary mutation either in the gal80 or GAL81 locus, whereas revertants due to mutation at the supposed controlling site for the structural gene cluster of the galactose-pathway enzymes have not been isolated.