FIGURE 5.

Single-stranded RNA loading to AGO-clade Argonaute proteins. (A) mir-451 is an miRNA widely conserved in vertebrates. Its precursor hairpin is generated by Drosha but too short (42 nt) to be cleaved by Dicer. Instead, the hairpin is directly loaded to Ago2 and cleaved on the 3′ arm by the slicer activity of Ago2. The resulting ac-pre-mir-451 (Ago2-cleaved-pre-miRNA: 30 nt) is further resected by the PARN exonuclease to mature as ~22–26 nt miR-451. (B) miRNA loop species loads to AGO. In the miRNA processing pathway, the loop region of pre-miRNA is released as non-structured RNA after processing by Dicer. Processed loops were considered as mere by-products, however, recent studies revealed that select miRNA loops are loaded to Argonaute complexes. Loaded loops are enriched with 5′U species, and their loading occurs independently of the HSP90/HSP70 system, in contrast to mature miRNAs loaded as duplexes. (C) Processing of tRNAs in eukaryotes. tRNAs are transcribed by RNA pol III whose transcription terminates with runs of U. The precursor tRNA contains a 5′ leader and a 3′ trailer, which are cleaved by RNaseP and RNaseZ, respectively. Nucleotidyl transferase adds CCA untemplated trinucleotides and the tRNA matures after further chemical modifications and aminoacylation. In addition to mature tRNA products, a variety of small RNA fragments (tRFs) are produced from tRNAs. After RNaseZ-mediated cleavage, some 3′ trailers containing 5′ Us are released as stable species termed 3′ U tRFs. Mature tRNAs can also produce Dicer-dependent species that are cleaved at the D-loop (5′ tRFs) or at the T-loop (3′ CCA tRFs). Some of 3′U tRFs, 5′ tRFs, and 3′ CCA tRFs are loaded to Argonaute complexes and play gene regulatory roles.