Résumé
RNA-directed DNA methylation, one of several RNA interference–mediated pathways in the nucleus
1
, has been documented in plants
2
,
3
and in human cells
4
,
5
. Despite progress in identifying the DNA methyltransferases, histone-modifying enzymes and RNA interference proteins needed for RNA-directed DNA methylation
1
, the mechanism remains incompletely understood. We screened for mutants defective in RNA-directed DNA methylation and silencing of a transgene promoter in
Arabidopsis thaliana
and identified three
drd
complementation groups
6
. DRD1 is a SNF2-like protein
6
required for RNA-directed
de novo
methylation. We report here that DRD2 and DRD3 correspond to the second-largest subunit and largest subunit, respectively, of a fourth class of DNA-dependent RNA polymerase (polymerase IV) that is unique to plants. DRD3 is a functionally diversified homolog of NRPD1a or SDE4, identified in a separate screen for mutants defective in post-transcriptional gene silencing
7
,
8
. The identical DNA methylation patterns observed in all three
drd
mutants suggest that DRD proteins cooperate to create a substrate for RNA-directed
de novo
methylation.