Comparing transcriptional initiation by RNA polymerases I and III.

We comment on the current understanding of transcriptional initiation by RNA polymerases I and III, and look for common modes of operation of these enzymes, emphasizing selected recent developments. These include definitive experiments on the constitution of the human RNA polymerase I transcription factor SL1/TIF-IB, the development of a genetic system for analyzing the function of RNA polymerase I in yeast, the elucidation of the structure of the human snRNA gene transcription factor SNAPc, and initial stages of mapping the protein-protein interactions involved in the assembly of transcriptional initiation complexes.