required to maintain repression2 is a novel protein that facilitates locus-specific paramutation in maize

Meiotically heritable epigenetic changes in gene regulation known as paramutations are facilitated by poorly understood trans-homolog interactions. Mutations affecting paramutations in maize (Zea mays) identify components required for the accumulation of 24-nucleotide RNAs. Some of these components have Arabidopsis thaliana orthologs that are part of an RNA-directed DNA methylation (RdDM) pathway. It remains unclear if small RNAs actually mediate paramutations and whether the maize-specific molecules identified to date define a mechanism distinct from RdDM. Here, we identify a novel protein required for paramutation at the maize purple plant1 locus. This required to maintain repression2 (RMR2) protein represents the founding member of a plant-specific clade of predicted proteins. We show that RMR2 is required for transcriptional repression at the Pl1-Rhoades haplotype, for accumulation of 24-nucleotide RNA species, and for maintenance of a 5-methylcytosine pattern distinct from that maintained by RNA polymerase IV. Genetic tests indicate that RMR2 is not required for paramutation occurring at the red1 locus. These results distinguish the paramutation-type mechanisms operating at specific haplotypes. The RMR2 clade of proteins provides a new entry point for understanding the diversity of epigenomic control operating in higher plants.