SCH9, a putative protein kinase from Saccharomyces cerevisiae, affects HOT1-stimulated recombination

HOT1 is a mitotic recombination hotspot derived from yeast rDNA. To further study HOT1 function, irans-acting H OT1 recombination mutants (hrm) that alter hotspot activity were isolated. hrm2-l mutants have decreased HOT1 activity and grow slowly. The HRM2 gene was cloned and found to be identical to SCH9, a gene that affects a growth-control mechanism that is partially redundant with the cAMP-dependent protein kinase A (PKA) pathway. Deletion of SCH9 decreases HOT1 and rDNA recombination but not other mitotic exchange. Although high levels of RNA polymerase I transcription initiated at HOT1 are required for its recombination-stimulating activity, sch9 mutations do not affect transcription initiated within HOT1. Thus, transcription is necessary but not sufficient for HOT1 activity. TPK1, which encodes a catalytic subunit of PKA, is a multicopy suppressor of the recombination and growth defects of sch9 mutants, suggesting that increased PKA activity compensates for SCH9 loss. RAS2 ^val19, which codes for a hyperactive RAS protein and increases PKA activity, suppresses both phenotypic defects of sch9 mutants. In contrast to TPK1 and RAS2 ^val19, the gene for split zinc finger protein 1 (SFP1) on a multicopy vector suppresses only the growth defects of sch9 mutants, indicating that growth and HOT1 functions of Sch9p are separable. Sch9p may affect signal transduction pathways which regulate proteins that are specifically required for HOT1-stimulated exchange.