The ratio of the proteinase B inhibitors IB1 and IB2 from baker's yeast was shown to depend on the yeast strain by specific immunoprecipitation from boiled yeast extract and subsequent electrophoresis of the heat-dissociated precipitates on polyacrylamide gels. Bothe IB1 and IB2 were found, IB2 being by far predominant. Saccharomyces carlsbergensis NCYC 74 contained IB1, whereas in Saccharomyces cerevisiae X 2180 only IB2 was present. When cells of the latter strain were labelled with [14C] leucine from the beginning of growth and pulsed with [3H] leucine during the stationary phase, no short-lived IB1 could be detected. However, the peak of IB2 resolved on the gel showed an increased 3H/14C ratio in comparison to the majority of the other cellular proteins. The increased 3H/14C ratio was found to be the result of catabolite repression of inhibitor synthesis during exponential growth: cells growing on glucose as carbon source contain high inhibitor levels only during the stationary phase of growth, whereas during growth on acetate high amounts of inhibitor are present even in exponentially growing cells. During the stationary phase of growth the inhibitor is degraded with the same half-life as the total cellular proteins (about 50 h).
Fungal Proteins Hot Temperature