Functional analysis of mutations in the PIS gene, which encodes Saccharomyces cerevisiae phosphatidylinositol synthase.

The PIS gene for an enzyme phosphatidylinositol synthase having an increased Km for myo-inositol, was isolated from Saccharomyces cerevisiae. The mutant PIS gene contained a CAA codon at position 114 instead of the CAC codon observed in the wild-type gene, resulting in alteration of the amino acid from His to Gln. Oligonucleotide mediated site-directed mutagenesis of PIS at codon 114 revealed that mutant genes with codons for Ala, Thr and Leu could support yeast cell growth in vivo, but those for Asp, Lys and Tyr could not. All mutant enzymes when expressed in Escherichia coli showed greatly reduced in vitro activity.