Overview of Post-Transcriptional RNA Splicing
Prokaryotic DNA
transcription
produces
messenger RNA, which is necessary for transfer from the cell nucleus to the
cytoplasm where translation occurs. In
contrast, eukaryotic DNA
transcription
takes place in a cell's nucleus and produces what is called a primary RNA
transcript or pre-messenger RNA. Before eukaryotic products of
transcription can be moved into the cytoplasm, they must undergo modifications
that allow them to become mature messenger RNA. Splicing is the name given
to the reaction that removes unnecessary segments of the primary RNA transcript,
called introns. The removal of the introns produces mRNA (see the figure,
below). Messenger RNA contains only exons, those portions of the primary RNA
transcript that will be translated into a protein.
Figure 2.1: Splicing Product
Unlike the sequence of an exon, intron sequences are unimportant. Only small
portions of an intron sequence are preserved. These portions, located near the
end of each intron, serve to identify a sequence as an intron, identifying the
sequence for removal. There are intron identifying portions:
- The 5' splice site, consisting of a guanine next to a uracil base at the
5' end of the intron.
- The 3' splice site, consisting of an adenine next to a guanine at the 3'
end of an intron.
- The branch point A, located about 30 nucleotides from the 3' end,
consisting of just one adenine.
Figure 2.2: Conserved Intron Sequences
With the help of the spliceosome, a multi-component protein, the splicing
reaction occurs in two steps. The spliceosome contains five small nuclear
ribonucleoproteins (snRNPS, pronounced "snurps"). They are called U1, U2, U4,
U5, and U6. Each snRNP contains protein components that are critical for the
splicing reaction. U1 binds directly to the 5' splice site via complementary
base pairing. U1 then recruits U2, which forms a complex with branch point A.
U4 and U6 work in concert to form a "pre-splicing complex" and U5 helps to hold
the exons in place between the first and second steps in the splicing reaction.
Once the splicing reactions have occurred and the exons have been joined, the
resulting mRNA is freed from the spliceosome machinery and the different snRNP
components are recycled for further use.
In addition to the post-transcriptional modifications already discussed (5' cap,
poly A tail addition, and splicing), a fourth type of modification can be made:
RNA editing. RNA editing is a modification that changes the mRNA sequence and
as a result alters the protein produced by that mRNA. Editing can occur in two
ways. First, by changing one nucleotide to another, and second by inserting or
deleting a nucleotide or nucleotides.
