Figure 2.

Model of the 21U-RNA biogenesis and novel factors required for each process. C. elegans 21U-RNA loci are divided into two groups, type I and type II, according to the presence of the upstream conserved 8-nt motif, which is recognized by FKH family transcription factors. Type I 21U-RNA loci have this motif and are clustered on chromosome IV. On the other hand, type II 21U-RNA loci lack the upstream motif and frequently present in both directions on the genome. The YR motif (Y = C or T; R = A or G) is important for efficient transcription initiation. Each 21U-RNA locus produces ∼26-nt capped 21U-RNA precursors. After initial processing, including decapping and removal of 2 nt at the 5′ end, the precursor RNA is thought to be loaded into PRG-1, and its 3′ end is further processed by trimming. The newly identified 21U-RNA biogenesis factors (green) and the steps where they function are shown.