Plant homeodomain fingers form a helping hand for transcription

Several recent publications demonstrate a co-activator function for a subgroup of plant homeodomain fingers, which in humans comprises PHF2, PHF8 and KIAA1718. Besides an N-terminal plant homeodomain (PHD) these proteins also harbor an enzymatically active Jumonji-C domain (JmjC). While they have been shown to bind via their PHDs to H3K4me3-bearing nucleosomes at active gene promoters, their JmjC-domains are able to remove mono- and dimethyl-lysine 9 or 27 on histone H3, and monomethyl-lysine 20 on histone H4, chromatin modifications which correlate with transcriptional repression. Such dual histone crosstalk insures the proper removal of repressive histone marks following transcriptional activation by RNA polymerases I and II. Mutations in the PHF8 gene lead to X-linked mental retardation (XLMR) and knockdown of KIAA1718 and PHF8 homologs in zebrafish causes brain defects. Thus, the co-activator function of this new class of chromatin modifying enzymes has important functional roles in neuronal development. To continue with the nomenclature for histone demethylases, we propose the usage of KDM7A, -B and -C for KIAA1718, PHF8 and PHF2 proteins, respectively.