New research published in Cell shows that switching on and off members of a family of adhesion genes in Saccharomyces cerevisiae is linked to morphological development.

The FLO gene family of S. cerevisiae encodes cell-wall glycoproteins. FLO11 is usually the only FLO gene expressed in the strain used by Halme et al. Four silent FLO genes, FLO1, 5, 9 and 10, are located in subtelomeric regions. FLO11 expression is required for surface adherence, filament formation and sliding motility. Expression of different individual FLO genes radically alters cell-adhesion properties. Variation in surface-gene expression is a common tactic used by pathogens to evade the host immune response. Now, Halme et al. show that genetic and epigenetic mechanisms vary FLO gene expression.

Using immunofluorescence and a transcriptional reporter, Halme et al. showed that FLO11 expression was variegated — some cells express it, others don't. Pedigree analysis showed that switching was reversible. Moving FLO11 to a new location prevented switching — showing that FLO11 switching was position-dependent, but switching was also shown to be FLO-promoter specific. The authors suggest that Sfl1p recruits Hda1p to silence FLO11. So, FLO11 regulation is both promoter-dependent and position-specific — this was a surprise because most previously characterized position-silencing-dependent switches are promoter-independent. Plus, because FLO11 is ∼46 kb from the telomere, this is the third example in S. cerevisiae of a telomere-independent silencing mechanism — it might be more widespread in yeast than previously thought.

What about the reservoir of silent FLO genes? Mutations in the IRA1 and IRA2 genes — the yeast Ras GTPase-activating proteins — which are inherently unstable, unlike the rest of the yeast genome, switch on and off silent FLO loci. Once silent FLO genes are switched on, epigenetic regulation imposes flexibility onto the phenotype-switching mechanism.

In diploid cells, switching FLO11 expression on and off correlated with the ability to form filaments. Epigenetic and genetic control of FLO expression and morphology could be important in pathogenesis — in Candida albicans white-opaque phenotype switching is necessary for full virulence. Switching might allow members of the colony to sample new niches, without committing each cell to a new, and potentially less advantageous, phenotype.