Changing directions in Pol III transcription.

Changing directions in Pol III transcriptionApproximately 8 years ago, our thinking about the organization of promoter elements in DNA took a sudden turn.The realization that initiation of transcription by RNA polymerase III relied upon intragenic sequences, and thus differed significantly from the paradigm then provided by prokaryotic RNA polymerases, raised many questions about the origins of the various transcription systems and why they might be so different.These questions are slowly being answered.Comparisons of the structures of RNA polymerases of eukaryotes and of prokaryotes are revealing their conserved functional elements, indicating they have common origins (for review, see Buhler et al. 1987).Concurrently, some of the ostensibly major differences between eukaryotic RNA polymerase II promoter elements and prokaryotic promoters (notably, the importance of far upstream sequences) no longer appear to be that distinctive (see Ptashne 1986).And now, an ever-increasing volume of information is dispelling the notion that promoter elements recognized by any of the eukaryotic RNA polymerases are necessarily unique.