Yeast flocculation: Reconciliation of physiological and genetic viewpoints

Abstract

Yeast flocculation 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 walls bind to sugar residues of neighbouring cell walls. These sugar receptors are intrinsic to the mannan comprising 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 (Flo1 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 structural gene for mannospecific flocculation is common or possibly ubiquitous in non-flocculent strains and in consequence, FLO1, FLO5 and FLO8 are probably regulatory genes, exerting positive control over the structural 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 and an association between L double-stranded RNA and the Flo1 phenotype.

Constitutive Flocculation inSaccharomyces cerevisiae Through Overexpression of theGTS1 Gene, coding for a ‘Glo’-type Zn-finger-containing Protein

“…The sensitivity of flocculation to mannose and/or glucose is explained by the lectin theory, which states that lectin-like protein on one cell binds in a Ca-dependent way to sugar receptors on a neighbouring cell. The MS group corresponds to the Flo1 phenotype, while the GMS group corresponds to the NewFlo phenotype (reviewed by Stratford, 1992). The FLO1 gene is one constituent of the Flo1 phenotype (Teunissen et al, 1993;Watari et al, 1994).…”

“…Flocculation determined by the FLO5 (Bidard et al, 1994) and FLO8 genes also belong phenotypically to the Flo1/MS group (Masy et al, 1992). No genetic determinants have been described or cloned for the NewFlo group, although it is anticipated that the gene(s) involved should code as well for a lectin-like protein (Stratford, 1992). Little is known about the mechanism of flocculation in MI strains.…”

Applications of yeast flocculation in biotechnological processes

“…Further work is required to elucidate the FLO2 function. Some controversial data has involved the dominant FLO8 gene [87][88][89][90][91][92][93]. Finally, Kobayashi and collaborators [94] cloned the FLO8 gene and their results suggested that the FLO8 gene mediates flocculation via transcriptional activation of the FLO1 gene.…”

Breeding of flocculent industrial alcohol yeast strains by self-cloning of the flocculation gene FLO1 and repeated-batch fermentation by transformants.

“…It was reported by Straver et al (1994) that two flocculation phenotypes were distinguished by sugar inhibition, and further, they suggested the existence of at least two distinct lectin-encoding flocculation genes. To date, many genetic studies on flocculation in S. cerevisiae have reported several chromosomal genes affecting flocculation (for reviews, see Stratford, 1992;Watari et al,1994b).FLO1, which is one of the dominant flocculation genes, has been cloned and sequenced by three groups (Bidard et al, 1995;Teunissen et al, 1993;Watari et al, 1994b). Recently it was shown that FLO1 should be a structural gene coding for a cell surface protein, which has a key role in the mechanism of yeast flocculation, because of the localization of its product (Bidard et al, 1995), the similarity of the N-terminal sequence between the putative FLO1 product and a protein that was purified from flocculent yeast cell surface, and the stimulative effect on the flocculation ability of the yeast cells (Straver, 1994 Bunkyo-ku, Tokyo 113-8657, Japan (Received July 17, 1998; Accepted October 8, 1998) A nonflocculent industrial polyploid yeast strain, Saccharomyces cerevisiae 396-9-6V, was converted to a flocculent one by introducing a functional FLO1 gene at the URA3 locus.…”

“…It was reported by Straver et al (1994) that two flocculation phenotypes were distinguished by sugar inhibition, and further, they suggested the existence of at least two distinct lectin-encoding flocculation genes. To date, many genetic studies on flocculation in S. cerevisiae have reported several chromosomal genes affecting flocculation (for reviews, see Stratford, 1992;Watari et al,1994b).…”