Occurrence of proteinase a isoinhibitors in wild type yeast strains and commercial baker's yeast

Abstract

The occurrence of the proteinase A inhibitors 2 and 3 was investigated in wild type strains of Saccharomyces cerevisiae and Saccharomyces carlsbergensis as well as in several strains of commercial baker's yeast. Haploid and diploid strains of Saccharomyces cerevisiae contain only proteinase A inhibitor 3 whereas in Saccharomyces carlsbergensis only proteinase A inhibitor 2 is found. Strains of commercial baker's yeast contain either proteinase A inhibitor 3 or both inhibitors in a constant ratio of 1:3. Single cell cultures isolated from a strain of commercial baker's yeast also contain a mixture of the two inhibitors. Therefore, baker's yeast is not a mixture of two different cell types but the genome for both inhibitors is present in each single cell. In general, the results indicate that the occurrence of the two proteinase A inhibitors is determined genetically and, therefore, they may be called “isoinhibitors”.