Figure 1. Organization of the rRNA genes.

The repetitive nature of the rDNA is illustrated in an electron microscopic image of a yeast nuclear chromatin spread (‘Miller spread’). Progressively longer rRNAs (stained for associated proteins) emanate from the many pol I complexes as they transcribe the rDNA, beginning at the promoter (P) and finishing at the terminator (T). Beneath, a representative mammalian rDNA repeat is outlined (not drawn to scale). Each human rDNA repeat unit (GenBank accession number: U13369) of ~43 kb contains an intergenic spacer (IGS) of ~30 kb (grey), which contains the transcription regulatory elements, and ~13 kb of sequences encoding the precursor rRNA (yellow). The rRNA genes are present in a single transcription unit, transcribed by pol I to yield a 47S precursor rRNA that is, in part, co-transcriptionally processed and modified by methylation and pseudo-uridinylation to produce the mature 18S, 5.8S and 28S rRNAs. Pol I initiates transcription at the human rDNA promoter (P), which contains an essential core element from −45 to +18 relative to the start site (+1), and an upstream control element (UCE) from −156 to −107 [7]. A spacer-promoter (SP) upstream of the gene promoter directs pol-I-dependent transcription of short-lived transcripts of unknown function. Several transcription-termination elements are located at the 3′ end of the transcribed region of the rRNA genes (T) and immediately upstream of the rRNA gene transcription start site (T₀), between the spacer and gene promoters [3].