Functional variability in adhesion and flocculation of yeast megasatellite genes

Abstract

Megasatellites are large tandem repeats found in all fungal genomes but especially abundant in the opportunistic pathogen Candida glabrata. They are encoded in genes involved in cell-cell interactions, either between yeasts or between yeast and human cells. In the present work, we have been using an iterative genetic system to delete several C. glabrata megasatellite-containing genes and found that two of them were positively involved in adhesion to epithelial cells, whereas three genes controlled negatively adhesion. Two of the latter, CAGL0B05061g or CAGL0A04851g, are also negative regulators of yeast-to-yeast adhesion, making them central players in controlling C. glabrata adherence properties. Using a series of synthetic Saccharomyces cerevisiae strains in which the FLO1 megasatellite was replaced by other tandem repeats of similar length but different sequences, we showed that the capacity of a strain to flocculate in liquid culture was unrelated to its capacity to adhere to epithelial cells or to invade agar. Finally, in order to understand how megasatellites were initially created and subsequently expanded, an experimental evolution system was set up, in which modified yeast strains containing different megasatellite seeds were grown in bioreactors for more than 200 generations and selected for their ability to sediment at the bottom of the culture tube. Several flocculation-positive mutants were isolated. Functionally relevant mutations included general transcription factors as well as a 230 kb segmental duplication.