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.
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