A motif within SET-domain proteins binds single-stranded nucleic acids and transcribed and supercoiled DNAs and can interfere with assembly of nucleosomes

The evolutionary conserved SET domain is present in many eukaryotic chromatin-associated proteins, including some members of the trithorax (TrxG) group and the polycomb (PcG) group of epigenetic transcriptional regulators and modifiers of position effect variegation. All SET domains examined exhibited histone lysine methyltransferase activity, implicating these proteins in the generation of epigenetic marks. However, the mode of the initial recruitment of SET proteins to target genes and the way that their association with the genes is maintained after replication are not known. We found that SET-containing proteins of the SET1 and SET2 families contain motifs in the pre-SET region or at the pre-SET-SET and SET-post-SET boundaries which very tightly bind single-stranded DNA (ssDNA) and RNA. These motifs also bind stretches of ssDNA generated by superhelicall tension or during the in vitro transcription of duplex DNA. Importantly, such binding withstands nucleosome assembly, interfering with the formation of regular nucleosomal arrays. Two representatives of the SIUV39 SET family, SU(VAR)3-9 and G9a, did not bind ssDNA. The trx(Z11) homeotic point mutation, which is located within TRX SET and disrupts embryonic development, impairs the ssDNA binding capacity of the protein. We suggest that the motifs described here may be directly involved in the biological function(s) of SET-containing proteins. The binding of single-stranded nucleic acids might play a role in the initial recruitment of the proteins to target genes, in the maintenance of their association after DNA replication, or in sustaining DNA stretches in a single-stranded configuration to allow for continuous transcription.