Forkhead genes in transcriptional silencing, cell morphology and the cell cycle: Overlapping and distinct functions for FKH1 and FKH2 in Saccharomyces cerevisiae

The SIR1 gene is one of four specialized genes in Saccharomyces cerevisiae required for repressing transcription at the silent mating-type cassettes, HML alpha and HMRa, by a mechanism known as silencing. Silencing requires t he assembly of a specialized chromatin structure analogous to heterochromat in. FKH1 was isolated as a gene that, when expressed in multiple copies, co uld substitute for the function of SIR1 in silencing HMRa. FKH1 (Forkhead H omologue One) was named for its homology to the forkhead family of eukalyot ic transcription factors classified on the basis of a conserved DNA binding domain. Deletion of FKH1 caused a defect in silencing HMRa, indicating tha t FKH1 has a positive role in silencing. Significantly, deletion of both FK H1 and its closest homologue in yeast, FKH2, caused a form of least pseudoh yphal growth, indicating that the two genes have redundant functions in con trolling yeast cell morphology. By several criteria, fkh1 Delta fkh2 Delta- induced pseudohyphal growth was distinct from the nutritionally induced for m of pseudohyphal growth observed in some strains of S. cerevisiae. Althoug h FKH2 is redundant with FKH1 in controlling pseudohyphal growth, the two g enes have different functions in silencing HMRa. High-copy expression of CL B2, a G2/M-phase cyclin, prevented fkh1 Delta fkh2 Delta-induced pseudohyph al growth and modulated some of the fkh Delta-induced silencing phenotypes. Interestingly, deletions in either FKH1 or FKH2 alone caused subtle but op posite effects on cell-cycle progression and CLB2 mRNA expression, consiste nt with a role for each of these genes in modulating the cell cycle and hav ing opposing effects on silencing. The differences between Fkh1p and Fkh2p in vivo were not attributable to differences in their DNA binding domains.