rRNA Synthesis and Processing

The genes coding for rRNA (except 5S rRNA) are located in the nucleolar part of the nucleus. The rRNA genes are highly repetitious and mammalian cells contain 100 to 2000 copies of the rRNA genes per cell. The genes are organised in transcription units separated by non-transcribed spacers. Each transcription unit contains sequences coding for 18S, 5.8S and 28S rRNA.
The transcription units are transcribed by RNA polymerase I into giant RNA molecules, primary transcripts, that in addition to the sequences corresponding to 18S, 5.8S and 28S rRNA contains external and internal transcribed spacer sequences. The rate of nucleolar transcription is very high and many polymerases operate on the same transcription unit. The transciptionally active DNA therefore has a Christmas tree-like appearance on electron microscopic pictures.

 electron micrograph»
  role of rRNA in translation»

The primary transcript is processed into the mature 18S, 5.8S and 28S rRNAs. The processing involves exo- and endo-nucleolytic cleavages guided by snoRNA (small nucleolar RNAs) in complex with proteins. The mature rRNAs contain modified nucleotides which are added after transcription by a snoRNA-dependent mechanism.

5S ribosomal RNA is transcribed by RNA polymerase III in the nucleoplasm. Each eukaryotic cell contains a high number of copies of the 5S coding gene (up to 20 000 copies per cell). 5S rRNA contains overlapping binding sites for two different proteins, ribosomal protein L5 and transcription factor TFIIIA. The mutual exclusive binding of these two proteins to 5S rRNA is important for coordinating the expression of 5S rRNA to the production of the other rRNAs.