SET-9 and SET-26 impact transgenerational epigenetic inheritance and germline mortality through their PHD finger and SET domains [small RNA-seq]
Transgenerational epigenetic inheritance (TEI), the transmission of epigenetic states between generations, has been observed in complex organisms ranging from plants to nematode worms, insects and mammals. One form of TEI that has been robustly studied is the inheritance of gene silencing. In Caenorhabditis elegans transgenerational gene silencing can be partitioned into three phases: the initiation of silencing, establishment of a heritable silencing signal within the parent individuals, and maintenance of this silencing signal within subsequent generations. Silencing involves nuclear and cytoplasmic components, and most genes involved in the process are linked to small RNA processing pathways. To date, two predicted histone methyltransferases have been implicated in the establishment of TEI, set-25 and set-32. Here, we demonstrate that the closely related paralogues set-9 and set-26 function in establishment and maintenance of TEI. We also identify a third related gene that lacks any predicted ordered domains; we name this gene intrinsically disordered protein-1 (idp-1). Strikingly, idp-1 also plays a role in establishment of TEI. SET-9 and SET-26 each contain a PHD finger and a SET domain. We show that the SET domains of SET-9 and SET-26 likely have no histone lysine methyltransferase activity, but that the PHD fingers bind trimethylated histone 3 lysine 4 (H3K4me3) in the absence of adjacent histone marks. We demonstrate that despite modelling data that suggests that set-9 and set-26 lack histone lysine methyltransferase activity, the function of both genes in TEI requires their PHD finger and SET domains. Our data confirm that the SET domain is functional and identifies three new chromatin factors required for epigenetic inheritance. Overall design: small RNA-seq libaries were prepared from gonads dissected from adult hermaphrodite C. elegans strains that carry set-9, set-26, and set-9;set-26 mutations, as well as the wild type (N2) strain.