570
chapter 25
RNA and Protein Synthesis
F I G U R E 2 5 - 8
Schematic drawing showing production of eukaryotic mRNA. The primary transcript is capped before it is released.
Then its 3'-OH end is modified, and finally the intervening regions are excised. MeG, 7-methylguanosine. **Two
nucleotides whose riboses may be methylated.
terminus is synthesized by a nuclear enzyme, poly(A)
polymerase. However, adenylate residues are not added
directly to the 3' terminus of the primary transcript. In-
stead, RNA polymerase transcribes past the recognition
site for addition of poly(A). After synthesis of the com-
plete RNA, endonucleolytic cleavage occurs at the poly(A)
recognition site in the RNA, and poly(A) is added. A se-
quence, AAUAAA, located 10-25 bases upstream from
the poly(A) addition site, is also required for enzyme
recognition of the site of polyadenylylation.
Splicing o f RNA in Eukaryotes
A
distinguishing feature of most primary transcripts of
higher eukaryotes is the presence of untranslated interven-
ing sequences (
introns
) that interrupt the coding sequence
and are excised from the primary RNA transcript. In the
processing of RNA in higher eukaryotes, the amount
of discarded RNA ranges from 30% to nearly 90% of
the primary transcript. The remaining coding segments
(exons)
are joined together by splicing enzymes to form
translatable mRNA molecules. The excision of the introns
and the formation of the final mRNA molecule by joining
of the exons is called RNA splicing. The 5' segment (the
cap) of the primary transcript is never discarded and hence
is always present in the completely processed mRNA
molecule; the 3' segment is also usually retained. Thus,
the number of exons is usually one more than the number
of introns. The number of introns per gene varies consid-
erably (Table 25-1). Furthermore, within different genes
the introns are distributed differently and have many sizes
(Figure 25-9), and introns are usually longer than exons.
The splicing reaction is remarkably precise: cuts are
made at unique positions in transcripts that contain
thousands of bases. The fidelity of the excision and splic-
ing reaction is extraordinary, for if an error of even one base
were made, the correct reading frame would be destroyed.
Such fidelity is achieved by recognition of particular base
sequences by splicing enzymes.
Base sequence studies of the regions adjacent to several
hundred different introns indicate that common sequences
can be found at each end of an intron. The sites at which
cutting occurs are always 5' to GU and 3' to AG. The rule
is that the base sequence of an intron begins with GU and
ends with AG.
Introns are excised one by one, and ligation occurs be-
fore the next intron is excised; thus, the number of different
nuclear RNA molecules present at any instant is huge.
Translation does not occur until processing is complete.
T A B L E 2 5 -1
Translated Eukaryotic Genes in Which Introns Have
Been Demonstrated
Gene
Number of Introns
a-Globulin
2
Immunoglobulin L chain
2
Immunoglobulin H chain
4
Yeast mitochondrial cytochrome
b
6
Ovomucoid
6
Ovalbumin
7
Ovotransferrin
16
Conalbumin
17
a-Collagen
52
*At present the histone and interferon genes are the only known
translated genes in the higher organisms that do not contain introns.
