Sulfate Uptake in Saccharomyces cerevisiae : Biochemical and Genetic Study

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Abstract
Sulfate uptake is the 1st step of the sulfate assimilation pathway, which was shown to be part of the methionine biosynthetic pathway. Kinetic study of sulfate uptake showed a biphasic curve in a Lineweaver-Burk plot. The analysis of this plot indicates that 2 enzymes participate in sulfate uptake: permease I has a high affinity for the substrate (Km = 0.005 mM); permease II shows a much lower affinity for sulfate (Km = 0.35 mM). Regulation of the synthesis of both permeases is under the control of exogenous methionine or S-adenosylmethionine. Synthesis of sulfate permeases is coordinated with the synthesis of the other methionine biosynthetic enzymes thus far studied. An additional specific regulation of sulfate permeases by inhibition of their activity by endogenous sulfate and adenosyl phosphosulfate (an intermediate metabolite in sulfate assimilation) was shown. A mutant unable to concentrate sulfate was selected. This strain carried mutations in 2 independent genes. These 2 mutations, separated in 2 different strains, lead to modified kinetics of sulfate uptake. The study of these strains suggested that there is an interaction in situ between the products of these 2 genes.