Structural Organization and Regulation of Transcription by RNA Polymerase I of Plant Nuclear Ribosomal RNA Genes

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The RNA components of the plant cytoplasmic ribosomes consist of the 17/18S rRNA (ribosomal RNA) in the 40S ribosome subunit and the 5S, 5.8S, and 25S/26S rRNA in the 60S ribosome subunit. The corresponding genes for the 18S, 5.8S and 25S rRNA, encoded by the nuclear genome, are composed in transcription units which are located as rDNA (ribosomal DNA) repeats in the NOR (nucleolus organizing region) of the chromosome. As in higher animals, the genes for the 5S rRNA are localized separately at other regions in the genome (Hemleben and Grierson 1978). Coordinated regulation of the expression of the different components of the ribosomes can be expected since three RNA polymerases are involved to provide concomitantly the rRNA components and the mRNA for the ribosomal proteins (Sommerville 1986): RNA polymerase I (pol I) is responsible for the 18S–25S rRNA transcription, RNA polymerase III (pol III) produces the 5S rRNA, and the genes for the ribosomal proteins are transcribed by RNA polymerase II (pol II). The question how these different RNA polymerases are coordinately regulated is still a fascinating problem to solve for eukaryotic cells. Until recently, it was believed that the genes transcribed by pots I, II and III, respectively, utilize completely different sets of initiation factors; however, it has now become evident for animal cells that one common factor, the TATA-binding protein, plays a central role in transcription of all three RNA polymerases (for a review, see White and Jackson 1992).