The TELOMERE REPEAT BINDING proteins TRB4 and TRB5 function as transcriptional activators of PRC2-controlled genes to regulate plant development
TELOMERE REPEAT BINDING proteins (TRBs) are plant specific transcriptional regulators that combine two DNA binding domains: the GH1 domain shared with H1 linker histones and the Myb/SANT domain that specifically recognize AAACCCT, the telomeric motif, present at telomeres, centromere-proximal Interstitial Telomeric Repeats (ITRs) and at promoters of numerous protein-coding genes. TRB1, TRB2 and TRB3 (TRB1-3) proteins collaborate with PRC2 and JMJ14 for deposition of H3K27me3 and removal of H3K4me3 to induce transcriptional repression in a functionally redundant manner.  Here we report that TRB4 and TRB5, which belong to a separate TRB clade conserved among spermatophytes, fulfill chromatin roles distinct from TRB1-3. We show that TRB4 and TRB5 are involved in the transcriptional control of several hundred genes involved in developmental responses to environmental cues and in consequence TRB4 and TRB5 loss of function plants present developmental growth defects including delayed flowering. Most of the trb4 trb5 transcriptomic defects differ from those of trb1 trb2 trb3, suggesting a distinct mode of action at the chromatin level. Indeed, TRB4 binds to several thousand sites in the genome, mainly to TSS and promoter regions of transcriptionally active and H3K4me3-marked genes, but it does not affect global H3K4me3 levels. In contrast to TRB1-3, TRB4 is not enriched at H3K27me3-marked gene bodies, and combined loss of TRB4 and TRB5 affects H3K27me3 only at a small subset of genes. Yet, TRB4 physically interacts with the catalytically active subunit of somatic PRC2 complexes CURLY LEAF (CLF). Unexpectedly, loss of TRB4 and TRB5 partially suppresses developmental defects of clf mutant plants, likely through their role as positive transcriptional regulators of the key flowering genes SOC1 and FT. We further show that TRB4 and TRB1 share multiple target genes and reveal physical and genetic interactions between TRBs of the two distinct clades, altogether unveiling that TRB proteins engage in both positive and negative interplay with other members of the family to regulate plant development through PRC2-dependent and independent mechanisms. Overall design: To investigate the role of TRB4 and TRB5 in Arabidopsis we analysed H3K4me3 and H3K27me3 profile in trb45 mutant compared to WT ChIP_seq experiments for H3K4me3 and H3K27me3 in WT (Col0) and in trb45 mutants