The regulatory role of Mss11p in Ca2+-dependent flocculation and haploid invasive growth in Saccharomyces cerevisiae .
Michael C. Bester (1), Ricardo R. Cordero Otero (1), Isak S. Pretorius (2) and Florian F. Bauer (1)
(1) Institute for Wine Biotechnology, Stellenbosch University, Stellenbosch, ZA-7600, South Africa; (2) Australian Wine Research Institute, Urrbrae Glen Osmond, Adelaide, SA 5064, Australia
In liquid medium, Saccharomyces cerevisiae strains grow as individual, dispersed cells. Cells may, however, clump together asexually to form aggregates or flocs in a process referred to as flocculation. As a direct result the rate of cellular sedimentation is greatly increased. Flocculation requires the presence of Ca2+ ions in the growth medium and is triggered by environmental conditions of which the information of is relayed through intracellular signalling pathways affecting the expression of the flocculation ( FLO ) gene family. Some strains of S. cerevisiae, in particular the commonly used laboratory strain S288C, appear to have lost the ability to flocculate. In S288C this inability has previously been attributed to a nonsense point mutation in the FLO8 gene, which encodes a transcriptional activator of some of the FLO genes. Here we show that Mss11p also acts as a strong inducer of flocculation. Mss11p has previously been implicated in the regulation of starch degradation, the formation of pseudohyphae and haploid invasive growth through the regulaton of the STA2 and FLO11 genes, respectively. Our data indicate that Mss11p induces flocculation together with Flo8p, that FLO1 is the dominant target gene, and that MSS11 deletion leads to the abolishment of flocculation, even in the presence of multiple copies of FLO8. By expressing truncated forms of MSS11, we provide evidence that suggest that specific domains of Mss11p are critical for this induction of flocculation.