Nucleotide sequence requirements for specific initiation of transcription by RNA polymerase I.

The nucleotide sequence(s) specifying RNA polymerase I initiation has been investigated by studying the transcription of deleted and nondeleted mouse ribosomal RNA gene (rDNA) templates in vitro. The deletion of 5'-flanking sequences upstream from position -- 39 did not affect transcriptional activity, but removal of sequences between positions -- 39 and -- 34 resulted in a 90% decrease of rDNA transcription. The template activity was completely eliminated by the further deletion of nucleotides -- 33 to -- 13. It is concluded that sequences between -- 34 and -- 12, upstream from the transcribed region, represent an essential control region for the initiation of transcription in vitro. Therefore, this region may be functionally analogous to the T-A-T-A box of RNA polymerase II promoters. In addition to this control region, sequences located further upstream (between positions -- 45 and -- 169) may also exert some function in efficient transcription initiation as revealed by competition experiments between wildtype and mutant rDNA templates.