Identification and sequence analysis of the ribosomal DNA promoter region of Crithidia fasciculata.

We have identified the promoter region of the large ribosomal DNA repeat unit of Crithidia fasciculata by northern blotting and nuclear run-on analyses. These data show that transcription starts approximately 1 kb upstream of the 18S rRNA gene. S1 protection experiments and sequence analysis of this area resulted in a precise localization of the start site. We have been unable to identify conserved sequence element(s) by a direct comparison of the crithidial RNA polymerase I promoter region and similar promoter regions of other eukaryotes; not even to the promoter region of the more closely related kinetoplastid species, Trypanosoma brucei. The absence of homology within the primary sequence of the promoter region, which is also found in other eukaryotes, might explain the observed species specificity of in vivo and in vitro rDNA transcription, since this resides in the interaction of initiation factor(s) and the core promoter domain.