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.