10.5: Ribosomal RNA Processing in Eukaryotic Nuclei
In most eukaryotes, a large rRNA gene in most eukaryotes transcribes a 45S precursor transcript containing (from shortest to longest) 5.8S rRNA, 18SrRNA and 28S rRNA. The ‘S’ stands for Svedberg, the biochemist who developed the sedimentation velocity ultracentrifugation technique to separate molecules like RNA by size. The higher the S value, the larger the molecule and therefore the faster it moves through the viscous sugar gradient during centrifugation. RNA Polymerase I transcribes 45S precursor rRNAs (preRNAs) from multiple large transcription units in the genome (shown below).
The 45S pre-rRNA is processed by cleavage. The many copies (200-400!) of the 45S gene in eukaryotic cells might be expected, since making proteins (and therefore ribosomes) will be an all-consuming cellular activity. In humans, 45S genes (45S rDNA) are distributed among five acrocentric chromosomes (those that have a centromere very near one end of the chromosome). The 45S rDNA in chromosomes is packed in the nucleolus inside nuclei.
Because these genes are present in so many copies and organized into a specific region of chromatin, it is possible to visualize 45S transcription in progress in electron micrographs such as the ones below.
The term lampbrush came from the shape of the 45S genes in the process of transcription; the RNAs extending from the DNA template look like an old-fashioned brush used to clean the chimney of a kerosene lamp.
Multiple gene copies encode 5S rRNAs. However, unlike the 45S rRNA genes, 5S rRNA gene may be spread among many chromosomes (seven in Neurospora crassa , the bread mold). Or in the case of humans, 5S RNA gene copies are distributed along chromosome 1. The 5S rRNA genes are transcribed by RNA polymerase III with minimal posttranscriptional processing. As already noted, the promoters of the 5S genes are within the transcribed part of the genes, rather than upstream of their 5S transcription units.