Department of Molecular Genetics, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, New York 10461 Department of Biochemistry and Cell Biology, Stony Brook University, Stony Brook, New York 11794-52152
Received 20 December 2005/ Returned for modification 9 January 2006/ Accepted 27 January 2006
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 bur1
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
bur1
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 bur1
. 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 bur2
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.
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