The ERI-6/7 helicase acts at the first stage of an siRNA amplification pathway that targets recent gene duplications

Endogenous small interfering rnas (sirnas) are a class of naturally occuring regulatory rnas found in fungi,plants,and animals. some endogenous sirnas are required to silence transposons or function in chromosome segregation; however,the specific roles of most endogenous sirnas are unclear. the helicase gene eri-6/7 was identified in the nematode caenorhabditis elegans by the enhanced response to exogenous double-stranded rnas (dsrnas) of the null mutant. eri-6/7 encodes a helicase homologous to small rna factors armitage in drosophila,sde3 in arabidopsis,and mov10 in humans. here we show that eri-6/7 mutations cause the loss of 26-nucleotide (nt) endogenous sirnas derived from genes and pseudogenes in oocytes and embryos,as well as deficiencies in somatic 22-nucleotide secondary sirnas corresponding to the same loci. about 80 genes are eri-6/7 targets that generate the embryonic endogenous sirnas that silence the corresponding mrnas. these 80 genes share extensive nucleotide sequence homology and are poorly conserved,suggesting a role for these endogenous sirnas in silencing of and thereby directing the fate of recently acquired,duplicated genes. unlike most endogenous sirnas in c. elegans,eri-6/7-dependent sirnas require dicer. we identify that the eri-6/7-dependent sirnas have a passenger strand that is ~19 nt and is inset by ~3-4 nts from both ends of the 26 nt guide sirna,suggesting non-canonical dicer processing. mutations in the argonaute ergo-1,which associates with eri-6/7-dependent 26 nt sirnas,cause passenger strand stabilization,indicating that ergo-1 is required to separate the sirna duplex,presumably through endonucleolytic cleavage of the passenger strand. thus,like several other sirna-associated argonautes with a conserved rnaseh motif,ergo-1 appears to be required for sirna maturation. © 2011 fischer et al.