Genetic analysis and enzyme activity suggest the existence of more than one minimal functional unit capable of synthesizing phosphoribosyl pyrophosphate in Saccharomyces cerevisiae

The PRS gene family in Saccharomyces cerevisiae consists of five genes each capable of encoding a 5-phosphoribosyl-1(alpha)-pyrophosphate synthetase p olypeptide, To gain insight into the functional organization of this gene f amily we have constructed a collection of strains containing all possible c ombinations of disruptions in the five PRS genes. Phenotypically these dele tant strains can be classified into three groups: (i) a lethal phenotype th at corresponds to strains containing a double disruption in PRS2 and PRS4 i n combination with a disruption in either PRS1 or PRS3; simultaneous deleti on of PRS1 and PRS5 or PRS3 and PRS5 are also lethal combinations; (ii) a s econd phenotype that is encountered in strains containing disruptions in PR S and PRS3 together or in combination with any of the other PRS genes manif ests itself as a reduction in growth rate, enzyme activity, and nucleotide content; (iii) a third phenotype that corresponds to strains that, although affected in their phosphoribosyl pyrophosphate-synthesizing ability, are u nimpaired for growth and have nucleotide profiles virtually the same as the mild type. Deletions of PRS2, PRS4, and PRS5 or combinations thereof cause this phenotype, These results suggest that the polypeptides encoded by the members of the PRS gene family may be organized into two functional entiti es, Evidence that these polypeptides interact with each other in vivo was o btained using the yeast two-hybrid system. Specifically PRS1 and PRS3 polyp eptides interact strongly with each other, and there are significant intera ctions between the PRS5 polypeptide and either the PRS2 or PRS4 polypeptide s. These data suggest that yeast phosphoribosyl pyrophosphate synthetase ex ists in vivo as multimeric complex(es).