H3K36me3 is significantly more enriched at Set1-affected loci than at MLL1/2-affected loci in planarian stem cells

Histone H3 lysine 4 trimethylation (H3K4me3) is known to correlate with both active and poised genomic loci, yet many questions remain regarding its functional roles in vivo. We identify functional genomic targets of two H3K4 methyltransferases, Set1 and MLL1/2, in both the stem cells (X1) and differentiated tissue (Xins) of the planarian flatworm Schmidtea mediterranea. We show that, despite their common substrate, these enzymes target distinct genomic loci in vivo, which are distinguishable by the footprint each enzyme leaves on the chromatin template, i.e., the breadth of the H3K4me3 peak. Whereas Set1 targets are broadly associated with the maintenance of the stem cell population, MLL1/2 targets are specifically enriched for genes involved in ciliogenesis. These data not only confirm that chromatin regulation is fundamental to planarian stem cell function, but also provide evidence for post-embryonic functional specificity of H3K4me3 methyltransferases in vivo. To test this, we looked at the H3K4me3 chromatin profile in WW, X1 and Xins cell types upon knockdown of set1, mll1/2, as well as WT.