Schematic architecture of pol III-transcribed snoRNA and tRNA genes in nematodes. (a) Upper panel: schematic diagram of a tRNA gene and associated transcription factors used for transcription by RNA polymerase (Pol) III. The transcription start site (TSS, red arrow), the A and B box promoter elements (orange and red rectangles) and the oligo-dT transcription terminator are indicated. TBP is a subunit of TFIIIB; the TFC-1 and TFC-4 subunits of TFIIIC are also indicated. Lower panel: a Pol III-dependent snoRNA gene with its tRNA like promoter and presumed transcription factors. Ikegami and Lieb  found an increased amount of RNA-seq reads in the region marked as 5′ leader in the NPP-13 mutants compared with the control. (b) Left: schematic representation of the removal of the 3′ trailer of a precursor tRNA by the processing endonuclease RNase Z. Note that the internal A and B box promoter elements of the gene are represented in the tRNA as the dihydrouridine loop (D loop) and pseudouridine (Ψ loop). The latter comprises a structural element recognized by RNase Z. Right: a speculative potential structure of a Pol III-dependent pre-snoRNA.