Regulation of ribosomal gene transcription.

In eukaryotic cells, ribosomal RNA (rRNA) gene is transcribed by RNA polymerase I (pol I) in the nucleolus as a large (40 S-47 S) precursor RNA (pre-rRNA) (for reviews, see refs. [1,2]). The mature rRNA species (28 S, 18 S and 5.8 S) are formed after a series of specific endonucleolytic cleavage reactions (for a review, see ref. [3]). The rRNA gene (rDNA) is highly reiterated and is arranged in a tandem array in clusters of headto-tail repeats. Each transcription unit is separated from the next unit by an intergenic spacer that ranges in size from 2 kb to over 30 kb. The transcriptionally active rRNA genes can be visualized in electron micrographic spreads of nucleolar chromatin as structures resembling Christmas trees where the rRNA gene, functional RNA pol I molecules and growing RNA chains can be observed as discrete entities [4]. Ribosomal RNA accounts for