The properties of a new polymerase III transcription factor reveal that transcription complexes can assemble by more than one pathway.

We have resolved a previously unidentified factor (TFIIID) that is required for in vitro transcription of polymerase III templates. Our ability to resolve factor D from each of the other components of the transcription machinery (polymerase and transcription factors IIIB and IIIC) allowed us to test the capacity of these separated components to form stable complexes with tRNA genes. We find that none of the individual components binds detectably to tRNA genes, but that certain combinations of transcription factors do bind. Our results show that TFIIID is essential for binding and that formation of a full transcription complex can proceed by either of two different pathways.