Functional RNAs are produced through modification ofpre-RNA produced from transcription process except bacterial mRNAs which are assuch used for protein synthesis without any modification. These series ofmodification involving removal of introns by splicing are known as processingof RNA.
Processingof rRNA & tRNA: The processing of both rRNA & tRNA in prokaryotes andeukaryotes are similar. Eukaryotes – 4 species of rRNAs, three of which (28S,18S & 5.8S) are derived by cleavage from single long precursor transcript(pre-rRNA).Whereas the fourth(5S) is transcribed from a separate gene.
There are 3 rRNAsin prokaryotes(23S, 16S & 5S) which are also transcribed from a singlepre-rRNA transcript. In prokaryotic cell initial cleavage yield separate precursor of 3individual rRNAs which undergo secondary cleavage to produce functional forms. Whereasin eukaryotic cell(within nucleolus) initial cleavage near 5′ side of 5.8S rRNAresults in 2 segments(18S & 28S + 5.8S). Further cleavage producefunctional forms with 5.8S having hydrogen-bonded to 28S. Apart from this therewill be also some addition of methyl groups to the bases & sugar moietiesof specific nucleotides and conversion of some uridines to pseudouridines.
Similarly from pre-tRNAs individual tRNA are synthesised in bothprokaryotes & eukaryotes. In prokaryotes, some tRNA s are included inpre-rRNA transcripts. RNase P enzyme(ribozyme) involved in processing at 5′ end of pre-tRNAs.
Itconsists of RNA & protein molecules. Here RNA is responsible for catalyticactivity whereas proteins are required for maximal activity. 3′ end processing involves action of conventional protein RNase andaddition of a CCA terminus. As this CCA sequences are the site of proteinattachment, it is present in all tRNAs for protein synthesis. In addition approximately10% of bases of specific nucleotides are modified. Introns are spliced usingendonuclease from pre-tRNAs.Processing of mRNA in eukaryotes: In contrast to prokaryotes, pre-mRNA synthesised in nucleus ofeukaryotes are modified before transported to cytoplasm. Processing involvesmodification of both ends of initial transcript and removal of introns.
The C-terminaldomain(CTD) of RNA polymerase II serve as binding site for enzyme complexes. Whereaspolymerases I and III lack a CTD, so their transcripts are not processed bysame enzyme complexes. Processing at 5′ end – addition of 7-methylguanosine cap.After transcription of first 20-30 nucleotides, enzymes for capping arerecruited to pCTD.
Capping initiated by addition of a GTP in reverseorientation to 5′ terminal nucleotide. Followed by addition of methyl group tothis G residue & to ribose moieties of one or two 5′ nucleotides. This 5’cap helps stabilising & aligning on ribosomes duting protein synthesis. At 3′ end – cleavage of primary transcript downstream to 10-30nucleotides of highly conserved hexanucleotide(AAUAAA in mammalian cells) andaddition of a poly-A tail known as polyadenylation.Thus G-U rich residue is degraded.
References: The Cell by Cooper and Hausman, 5th edition.