On solid surfaces wild strains of Saccharomyces cerevisiae form biofilm-like, structured colonies enabling them to survive long-term in hostile environments in the wild. However, the molecular mechanisms underlying the spatio-temporal development of colonies and their resistance to hostile conditions are still largely unknown. In this study, we analyzed the effect of the HAC1 gene on the colony morphology of wild strains of S. cerevisiae. The transcription factor Hac1p activates the unfolded protein response (UPR), which leads to activation of the expression of genes encoding components of the protein secretory pathway, and genes involved in stress responses in the endoplasmic reticulum (ER). The impact of HAC1 deletion is significant even under non-stress conditions and causes a radical reduction of structured colony architecture in hac1∆ strains derived from two wild S. cerevisiae strains (PORT and BR-F-Flo11p-GFP) and one laboratory ΣSh strain forming semi-fluffy or fluffy colonies. The hac1∆ strains exhibit a decreased vegetative growth rate, reduced cell attachment to the agar and an ineffective cell-cell adhesion resulting in decreased flocculation. The hac1∆ strains derived from BR-F-Flo11p-GFP contain a low level of Flo11p surface adhesin which is considered very important for the proper...

FLO11 is coding for a cell surface adhesin in the baker s yeast Saccharomyces cerevisiae. Its expression is regulated by different environmental circumstances like glucose, nitrogen or amino acid limitation. Flo11p is strictly required to allow cells to react on these nutrient signals by a dimorphic switch from single growing yeast cells to multicellular complexes with adhesive phenotype. This work demonstrates that under repressed conditions the unusually large FLO11 promoter of about 3 kb contains only one MNase-sensitive site located 1.2 kb upstream of the open reading frame. This site correlates with the binding region for the repressor protein Sfl1p. Investigations with genes for components involved in chromatin establishment, maintenance or remodeling identified the histone variant H2A.Z/Htz1p as yet unknown factor that is required to keep FLO11 in a silent state. The chromatin remodeler Rsc1p and the histone acetyl transferase Gcn5p are antagonists to H2A.Z/Htz1p and are required to overcome this silent state under glucose depletion, and therefore, to switch to the adhesive growth mode or pseudohyphal development. Addition of the histidine analogue 3-aminotriazol results in amino acid starvation and restores Flo11p-dependent adhesion in rsc1 mutant cells. These cells express only low FLO11 mRNA levels suggesting that there m! ight be additional mechanisms which result in sufficient amounts of adhesin molecules. These mechanisms might be regulated on a post-transcriptional level. A possible post-transcriptional level of controlling FLO11 expression was addressed by analysing two isogenic ribosomal proteins, namely Rps26Ap and Rps26Bp. Both proteins are compounds of the small subunit of the ribosome and are involved in regulating FLO11 expression. Only Rps26Ap is an essential factor for efficient FLO11 mRNA translation. Investigations concerning the regulation of the two isogenes demonstrate a reciprocal effect on the translational level. Rps26Ap stimulates the translation of RPS26B mRNA into the protein, whereas formation of Rps26Ap is inhibited by Rps26Bp.