The zinc finger protein Gsf1 regulates Gsf2-dependent flocculation in fission yeast

Fission yeast flocculates nonsexually by induction of the flocculin encoded by gsf2⁺ which is controlled by the positive regulator Mbx2. Here, we report a novel gene designated gsf1⁺ found to be a negative regulator of nonsexual flocculation. We identified gsf1⁺ as a multicopy suppressor of a sam2 mutation, which caused growth sensitivity to Ca²⁺ and also found a nonsense mutation in gsf1⁺ in a previously isolated gsf1 mutant. The gsf1⁺ gene encodes a 547-aa protein containing a Zn(2)-Cys(6) binuclear cluster-type zinc finger motif. The Gsf1 protein localized in the nucleus, consistent with a role as a transcription factor. Deletion of gsf1⁺ resulted in nonsexual flocculation inducible by CaCl₂, which was suppressed by the addition of EDTA or galactose. Both gsf2⁺ and mbx2⁺ were highly expressed in the gsf1 mutant. gsf1{increment} gsf2{increment} and gsf1{increment} mbx2{increment} double mutants did not flocculate, suggesting that gsf1⁺ is an upstream regulator. In addition, the gsf1 mutant was sensitive to CaCl₂, KCl, HU, and TBZ, consistent with the possibility that gsf1⁺ plays a role in functions unrelated to flocculation. Taken together, these results suggest that nonsexual flocculation in fission yeast is negatively controlled by Gsf1, which controls expression of mbx2⁺ and gsf2⁺.