Rice Trithorax factor ULTRAPETALA 1 (OsULT1) specifically binds to "GAGAG" sequence motif present in Polycomb response elements.

Co-ordinated interplay between Polycomb group (PcG) and Trithorax group (TrxG) of proteins regulate chromatin state and maintain the transcription "off" and "on" state of a gene in higher eukaryotes. Targeting PcG complex to a specific locus is mediated by DNA sequences known as Polycomb response elements. Interestingly, these PREs are also recognized by TrxG proteins to antagonise PcG mediated gene repression. In this study, we have characterised DNA binding property of rice trithorax group factor ULTRAPETALA1 (OsULT1) which has a SAND domain and B-box motif. Chromatin immunoprecipitation assay indicates cold induced enrichment of OsULT1 occupancy and a decrease in H3K27me3 mark in the promoter region of OsDREB1b gene, during transcription activation. OsULT1 binds to the cis motif "GAGAG", and the sequence specificity is contributed mainly by the SAND domain. GAGAG is one of the cis motifs present in PREs that are recognized by Drosophila GAGA factor and Pipsqueak. Thus, binding of OsULT1 to GAGAG motif, along with a decrease in H3K27me3 suggests that OsULT1 antagonises the repressive effect of PcG complex for transcriptional activation of OsDREB1b. Moreover, OsULT1 interacts with rice SET domain-containing methyltransferase TRX1, suggesting OsULT1 is an integral part of plant Trithorax group complex. Furthermore, the increase in ULT1 levels during environmental cues suggests its involvement in the transcriptional regulation of stress responsive genes. Collectively, these results suggest that the antagonistic functions of PcG and TrxG proteins and the mechanism of recruitment of these complexes to target loci are evolutionarily conserved for gene expression regulation across kingdoms.