Down-regulation of the expression of PKC1 and SRB1/PSA1/VIG9, two genes involved in cell wall integrity in Saccharomyces cerevisiae, causes flocculation

The cell wall integrity determinants PKC1 and SRB1/PSA1/VIG9 of Saccharomyc es cerevisiae were expressed under the control of the tightly regulated pro moter pMET3. Substitution of the cell-cycle-regulated SRB1/PSA1 native prom oter with pMET3 led to faster cell growth, larger cell volumes, and a twofo ld reduction of the steady-state SRB1/PSA1 mRNA level. In addition, the new pattern of expression of SRB1/PSA1 resulted in a dominant flocculation phe notype at all phases of batch growth. By contrast, expression of PKC1 from pMET3 increased the flocculation capacity of cells only at stationary phase . Methionine-mediated repression of either PSA1/SRB1 or PKC1 resulted in en hanced cell clumping. Cells in which both these genes had been replaced wit h their respective pMET3-regulated cassettes were highly flocculent under b oth expression and repression conditions. These results suggest that greate r exposure of flocculin on the cell surface, caused by either cell wall dis tortion (through depletion of Pkc1p) or aberrant regulation of mannosylatio n (through constitutive production of Srb1p), results in an increased flocc ulation ability.