FIGURE 2.

A model of human transcription complex of Pol III assembled on intron-containing tRNA gene. The depicted tRNA gene has two main regulatory elements, Box A and Box B. These elements are bound by the transcription factors TFIIIB and TFIIIC that recruit Pol III. The polymerase initiates transcription from a start point (arrow) and terminates at a short poly-T sequence (TTTT), the terminator. Pol III reinitiates via a facilitated recycling mechanism by which it performs rounds of transcription without leaving the gene. Depending on the tRNA gene, a nascent precursor tRNA can have 5′ leader (red), short intron (green), and 3′ trailer of uridines. The positions of the polymerase, TFIIIB, TFIIIC, RNase P, tRNA splicing complex, RNase Z, La protein, and others (e.g., nucleotide modifiers, helicases, transport factors) are arbitrary.