The subunit structure of Saccharomyces cerevisiae transcription factor IIIC probed with a novel photocrosslinking reagent.

A photocrosslinking nucleotide, 5-[N-(p-azidobenzoyl)-3-aminoallyl]-deoxyuridine monophosphate (N3Rd-UMP), has been used to identify four polypeptides that are associated with the large Saccharomyces cerevisiae RNA polymerase III transcription factor TFIIIC, and to map the locations of these subunits along DNA when TFIIIC binds to the S.cerevisiae SUP4 tRNA(Tyr) gene. The 145 kd subunit of TFIIIC is primarily accessible to photocrosslinking from the vicinity of the box B + internal promoter element; 95 and 55 kd subunits are located on opposite sides of the DNA helix in the vicinity of the box A internal promoter element; a 135 kd subunit is less strongly crosslinked to the box A region and to a DNA segment between boxes B and A. DNA probes containing more than one N3RdUMP residue can form crosslinks between polypeptide chains. The specific circumstances of formation and the apparent mol. wts of two of these products lead to the tentative suggestion that a protomer of TFIIIC may contain two 95 kd subunits.