In all cases, the process starts with transcription factors binding to control sequences and ends with TFIIIB (Transcription Factor for polymerase III B) being recruited to the complex and assembling Pol III.
TFIIIB consists of three subunits: TATA binding protein (TBP), a TFIIB-related factor (BRF1, or BRF2 for transcription of a subset of Pol III-transcribed genes in vertebrates), and a B-double-prime (BDP1) unit.
[3] Typical stages in 5S rRNA (also termed class I) gene initiation: Typical stages in a tRNA (also termed class II) gene initiation: Typical stages in a U6 snRNA (also termed class III) gene initiation (documented in vertebrates only): TFIIIB remains bound to DNA following the initiation of transcription by Pol III, unlike bacterial σ factors and most of the basal transcription factors for Pol II transcription.
The presence of transcripts with five, six, and seven U residues and the slow readthrough of the T7 stretch suggest that the incorporation of a single G into the RNA chain served to reset elongation rates either entirely or substantially.
[10] After the transient RNA-DNA hybrid intermediate is formed, the RNA strand is replaced by the RAD51 protein, which then catalyzes the ssDNA invasion step of homologous recombination.