In this article we will discuss about the processing of rRNA in prokaryotes (e.g. E. coli) and eukaryotes (e.g. mammals).
In prokaryotes (e.g. E. coli) there are seven operons for rRNA. These are dispersed throughout the genome. Each operon consists of one copy of each of the 5S, 16S and 23S rRNA sequences. Coding sequences for tRNA molecules are present between one and four rRNA operons. The primary transcripts are processed to form both rRNA and tRNA.
The nascent transcript is short lived and about 6000 nucleotides long having sedimentation coefficient 30S. The primary RNA transcript folds into many stem-loop structures through base pairing between complementary sequences. This secondary structure of stem – loop allows some of the proteins to bind with it and form ribonucleoprotein (RNP) complex. The RNPs are the RNA – protein complexes.
Then a methyl group is added to the base adenine by a methylating agent, S-adenosylmethionine. Thereafter, RNase II carries out primary cleavage to from precursors of 5S, 16S and 23S molecules. Secondary cleavage of precursor is done by RNase M5, M16 and M23, respectively to form mature rRNA molecules.
In eukaryotes (e.g. mammals) the RNA genes exist in tandem repeated clusters containing 100 or more copies of transcriptional units. They are transcribed in the nucleolus by RNA Pol I to yield a long, single pre-rRNA molecule which contains one copy each of the 18S, 5.8S and 28S sequences. In each organism, the precursor has a characteristic size e.g. 7,000 nucleotides in yeast and 13,500 nucleotides in mammals.
Many spacer sequences are removed from the long mammalian pre-tRNA molecule (47S) by a series of specific cleavage, first in the external transcribed spacers (ETS) 1 and 2 and then in the internal transcribed spacers (ITS) to release 20S pre-rRNA from the 32S pre- rRNA molecule (Fig. 10.10). Further both of these precursors must be trimmed and the 5.8S region must base pair to the 28S rRNA before production of the mature molecules.
Methylation takes place at more than 100 sites to from 2′-0-methyl-ribose. This is carried out by small nuclear RNP (snRNP) particles which are abundant in the nucleolus. Maturing RNA molecules fold and complex with ribosomal proteins. RNA Pol III synthesizes the 5S rRNA from unlinked genes which undergoes a little processing.