Flocculation, the calcium-dependent, non-sexual aggregation of yeast cells into 'flocs', is stimulated by nutrient limitation and is a process of great importance to the brewing characteristics of yeast strains (3). Characterization of flocculation characteristics has defined two classes of flocculation phenotypes. The FLO1-type is inhibited only by mannose sugars (4). The NewFLO-type, common in brewing strains, is inhibited by the sugars mannose, maltose, glucose, and sucrose, as well as by ammonium ions (4).

Genetic experiments defined three dominant flocculation genes, FLO1, FLO5, and FLO8, which are all in the FLO1-type phenotypic class 3. The FLO1 and FLO5 phenotypes could be distinguished by differences in sensitivity to heat or chymotrypsin. Floc-forming ability conferred by FLO1 is chymotrypsin-sensitive and heat-resistant, while floc-forming ability conferred by FLO5 is chymotrypsin-resistant but heat-labile 1. The FLO8 phenotype, however, turned out to be allelic with FLO1 5; this FLO8 is unrelated to FLO8/YER109C, which is a transcription factor 6.

Flo1p is a lectin-like cell-surface protein which aggregates cells into 'flocs' by binding to mannose sugar chains on the surfaces of other cells 2. Both the phenotypic characterization of FLO5 strains 4 and the sequence similarity between FLO5 and FLO1 5 suggest that Flo5p is also a mannose-binding lectin-like cell surface protein. Comparison of an Lg-FLO gene, cloned from a bottom-fermenting commercial yeast strain with the NewFLO phenotype, with the FLO1 gene demonstrates that the Lg-FLO gene is largely similar to FLO1, with some differences in the sugar binding site in the N-terminus of each protein 2.

Sequence analysis suggests that the flocculation genes are a multi-gene family localized to telomeric sequences, similarly to the SUC, MEL, and MAL gene families, all of which also encode characteristics important for brewing strains (3). In addition to FLO1 and FLO5, there are thought to be two more flocculation genes and three pseudogenes. The FLO9 gene (YAL063C) is 94% similar to the FLO1 gene and may be the source of a 4.2-kb transcript whose presence is not affected by deletions of FLO1 or FLO5. The FLO10 gene (YKR102W) is 58% similar to FLO1, but has 81.9% similarity in the N-terminus (3). Three presumed pseudogenes, YAL065C, YAR061W and YAR062W, and YHR213W have also been identified as fragmentary pieces with sequence similarity to FLO1 (3).

Last updated: 2004-02-09