TELOMERE REPEAT BINDING PROTEINS assist Polycomb Group protein complexes in the repression of target genes. TRB1

In multicellular organisms, Polycomb Repressive Complex (PRC) 1 and 2 repress target genes through chromatin compaction 1–3. Arabidopsis mutants strongly compromised in the pathway cannot develop differentiated organs, while loss of LIKE HETEROCHROMATIN PROTEIN 1 (LHP1) causes relatively mild phenotypic alterations although LHP1 is the only plant PRC1 component shown to associate genome-wide with the H3K27me3 modification set by PRC2. We screened for genetic enhancers of lhp1 mutants to identify novel components repressing target genes together with or in parallel to LHP1. Two enhancer mutations were mapped by isogenic mapping-by-sequencing to TELOMERE REPEAT BINDING PROTEIN (TRB) 1 and the paralog TRB3. ChIPseq analysis revealed that TRB1 binds to thousands of genomic sites containing cis-elements related to the telobox and that the number of binding sites as well as the enrichment at binding sites increases significantly in lhp1 mutants. RNA-seq analysis supports a model in which TRBs act as general transcriptional repressors that uphold down-regulation of Polycomb Group (PcG) target genes when chromatin-mediated repression is compromised by mutation.