RNA Processing

After transcription, RNA in eukaryotes undergoes significant processing. Transcripts that specify proteins are modified in the nucleus by the addition of 7-methylguanine caps at their 5' ends and poly-A tails approximately 100-250 nucleotides long at their 3' termini. The pre-mRNA is converted into mRNA by the excision of introns and splicing of exons. Most splicing is carried out by enzyme complexes, called spliceosomes, in the nucleus. Spliceosomes consist of 4 different small nuclear ribonucleoprotein particles (snRNPs) that work together to bring the ends of exons in a primary transcript near each other (Figure 4-4). The snRNPs are constructed from six to ten proteins and one or two of the five small nuclear RNAs (snRNAs) designated U1, U2, U4, U5, and U6. The snRNPs are generally designated by the snRNAs they contain.

U1 snRNP binds to the 5' exon-intron junction, U5 snRNP attaches near the 3' intron-exon junction, whereas U4-U6 snRNP binds near U5, and U2 associates where a lariat branch point will form (Figure 4-4b,c). The spliceosome, in particular U1 snRNP, cuts at the 3' end of an exon (#5 as an example in Figure 4-4c). U2 snRNA catalyzes the formation of the lariat, whereas U5 catalyzes the cut at the 5' end of exon 6 and the splicing of exon 5 to exon 6 (Figure 4-4d).

In the simplest case, a spliceosome promotes the excision of an intron between two exons and the splicing together of the two exons. In more complicated cases, a spliceosome may promote alternative splicing, the splicing of a pre-mRNA into different combinations of targeted exons. The mRNAis subsequently transported to the cytoplasm where it is translated into proteins.