|MoBio||RNA Processing||Chapter 5|
RNA processing is to generate a mature mRNA (for protein genes) or a functional tRNA or rRNA from the primary transcript. In this section, we discuss first the processing of pre-mRNA and then processing of pre-rRNA and pre-tRNA.
Processing of pre-mRNA involves the following steps:
In some cases, RNA editing is also involved.
Capping occurs shortly after transcription begins. The chemical structure of the "cap" is shown in the following figure, where m7G is linked to the first nucleotide by a special 5'-5' triphosphate linkage. In most organisms, the first nucleotide is methylated at the 2'-hydroxyl of the ribose. In vertebrates, the second nucleotide is also methylated.
Figure 5-A-2. Modifications at the 5' end.
A stretch of adenylate residues are added to the 3' end. The poly-A tail contains ~ 250 A residues in mammals, and ~ 100 in yeasts.
Processing of pre-rRNA and pre-tRNA
The newly transcribed pre-rRNA is a cluster of three rRNAs: 18S, 5.8S and 28S in mammals. They must be separated to become functional. Pre-rRNA is synthesized in the nucleolus. The U3 snRNA, other U-rich snRNAs, and their associated proteins in the nucleolus are involved in the cleavage of the pre-rRNA.
5S rRNA is synthesized in the nucleoplasm. It does not require any processing. After 5S rRNA is synthesized, it will enter the nulceolus to combine with 28S and 5.8S rRNAs, forming the large subunit of the ribosome.
Pre-tRNA requires extensive processing to become a functional tRNA. Four types of modifications are involved: