INCREASED ACTIVITY OF WALL HYDROLYTIC ENZYMES MAY EXPLAIN THE PHENOTYPE OF SACCHAROMYCES-CEREVISIAE FRAGILE MUTANTS

Fragile mutants of Saccharomyces cerevisiae require osmotic stabilizer s and lyse in hypotonic solutions. A single recessive mutation, srb1, is responsible for their phenotype, but the cause of cell lysis remain s uncertain. We have analyzed three possible mechanisms for this behav ior: comparative amounts of wall per cell; their chitin content; and t he relative activity of wall hydrolytic enzymes activated by osmotic s hock. We found normal amounts of wall and higher amounts of chitin in the fragile mutants. Determination of lytic enzymes by radiolabel of t he reducing ends of wall polysaccharides gave results suggesting that fragile mutants produce increased amounts of stretch-activated wall hy drolytic enzymes, which may be responsible for their lysis in hypotoni c media. These enzymes normally may play a role in cell wall growth an d shaping.