Abstract
BUR1 and BUR2 encode the catalytic and regulatory subunits of a cyclin-dependent protein kinase complex that is essential for normal growth and has a general role in transcription elongation. To gain insight into its specific role in vivo, we identified mutations that reverse the severe growth defect of bur1Delta cells. This selection identified mutations in SET2, which encodes a histone methylase that targets lysine 36 of histone H3 and, like BUR1, has a poorly characterized role during transcription elongation. This genetic relationship indicates that SET2 activity is required for the growth defect observed in bur1Delta strains. This SET2-dependent growth inhibition occurs via methylation of histone H3 on lysine 36, since a methylation-defective allele of SET2 or a histone H3 K36R mutation also suppressed bur1Delta. We have explored the relationship between BUR1 and SET2 at the biochemical level and find that histone H3 is monomethylated, dimethylated, and trimethylated on lysine 36 in wild-type cells, but trimethylation is significantly reduced in bur1 and bur2 mutant strains. A similar methylation pattern is observed in RNA polymerase II C-terminal domain truncation mutants and in an spt16 mutant strain. Chromatin immunoprecipitation assays reveal that the transcription-dependent increase in trimethylated K36 over open reading frames is significantly reduced in bur2Delta strains. These results establish links between a regulatory protein kinase and histone methylation and lead to a model in which the Bur1-Bur2 complex counteracts an inhibitory effect of Set2-dependent histone methylation.