The extracellular glucoamylase from certain strains of Saccharomyces cerevisiae can be purified from culture medium by a simple chromatographic procedure. The native enzyme is heavily glycosylated and has an Mr of about 250,000, but gel filtration indicates the existence of oligomers of larger size. Dissociation yields a form of Mr about 70,000. The glucoamylase is rich in serine and threonine and in aspartic acid plus asparagine, and has a pI of 4.62 and a pH optimum of 4.5-6.5. The thermostability and resistance to denaturants of the yeast enzyme is compared with those of two other fungal glucoamylases. Kinetic data for the yeast enzyme and a variety of substrates is presented; the enzyme is particularly ineffective in cleaving alpha-(1----6)-glycosidic bonds.

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The stability of the 2 mu-based yeast plasmid pJDB248 in Saccharomyces cerevisiae S150-2B(cir0) was investigated in glucose-limited chemostat culture. Plasmid-free cells were detected by loss of (plasmid-encoded) leucine prototrophy and confirmed by colony hybridization. The plasmid was considerably more stable at a high dilution rate (0.12 h-1) than at a lower dilution rate (0.05 h-1). The average plasmid copy number in the cells retaining the plasmid remained constant at approximately 50 in the high dilution rate culture whereas it rose to almost 600 in the slow dilution rate culture. However, in both cultures the overall plasmid level in the total population remained constant, indicating that plasmid segregation breaks down at the low growth rate. Similar experiments on the native 2 mu plasmid demonstrated high stability and no significant differences between the high and low growth rate cultures. It is postulated that the difference in behaviour between the native and chimeric plasmids is related to an interaction between the growth conditions and the loss of the D gene product.

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