Life and Medical Sciences
Wiley InterScience Backfile Collection 1832-2000
Yeast floccultion results from surface expression of specific proteins (lectins). Two flocculation phenotypes were suggested by physiological and biochemical tests, whereas genetic data suggested a larger number of mechanisms of flocculation. After reviewing the biochemistry, physiology and genetics of flocculation, a new hypothesis combining the data available from these different sources, is proposed.Flocculation results when lectins present on flocculent cell bind sugar residues of neighbouring cell walls. These sugar receptors are intrinsic to the mannan comprizing cell walls of Saccharomyces cerevisiae. Two lectin phenotypes were revealed by sugar inhibition studies. The gluco- and mannospecific NewFlo phenotype is not, as yet, found in genetically defined strains. Mannospecific flocculation (Flo 1 phenotype) is found in strains containing the genes FLO1, FLO5 and FLO8. This phenotype is also found following mutation of the TUP1 or CYC8 loci, in previously non-flocculent strains. It is therefore proposed that the structure gene for mannospecific flocculation is common or possibly unbiquitous in non-flocculent strains and in consequence, FLO1, FLO5 and FLO8 are probably regulatory genes, exerting positive control over the structure gene.Flocculation expression requires lectin secretion to the cell surface. Many of the observed ‘suppressions’ of flocculation may be due to mutations of the secretory process, involved in transporting structural proteins to the cell wall.The possible involvement of killer L double-stranded RNA with flocculation is suggested, given the lectin properties of viral coat proteins nad an association between L double-stranded RNA and the Flo 1 phenotype.
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