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

The occurrence of the proteinase A inhibitors 2 and 3 was investigated in wild type strains of $\text{Saccharomyces}\text{cerevisiae}$ and $\text{Saccharomyces}\text{carlsbergensis}$ as well as in several strains of commercial baker's yeast. Haploid and diploid strains of $\text{Saccharomyces}\text{cerevisiae}$ contain only proteinase A inhibitor 3 whereas in $\text{Saccharomyces}\text{carlsbergensis}$ 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”.