Biology Reference
In-Depth Information
3
Introns, exons, and
evolution
3.1 Intron structure, function and evolution
Eukaryotic genes are not usually contiguous entities but are instead interrupted
by noncoding sequences termed introns. How were introns first acquired?
Although there is at present no clear unambiguous answer to this question, there
appears to be a continuous evolutionary line from the archaic self splicing group
II introns of eubacteria and simple eukaryotes (Belfort, 1993; Lambowitz and
Belfort, 1993) via protein-assisted self-splicing introns to the splicing exhibited
by extant eukaryotic nuclear protein-encoding genes. It is therefore reasonable to
speculate that group II introns could have been introduced by the invasion of
eukaryotic cells by the bacterial endosymbiotic ancestor of the mitochondrion
(Cavalier-Smith, 1991).
Whatever their origin, a close correlation exists between intron density and
developmental complexity. Thus introns are largely, although not exclusively,
confined to the eukaryotes. The archaebacteria, which form a third major taxon
distinct from the eukaryotes and eubacteria, possess small introns in their tRNA
and rRNA genes whilst eubacteria and a few eukaryotes possess introns that catal-
yse their own splicing. An average of one intron per kilobase (kb) of coding
sequence is found in simple eukaryotes such as
Dictyostelium
and
Plasmodium
, 3-4
per kb in plants and fungi and 6 per kb in vertebrates (Palmer and Logsdon,
1991). The average cellular gene in vertebrates contains about seven introns
(Sharp, 1994). Intron size also increases with phylogenetic complexity with
intronic sequence accounting for only 10-20% of primary transcripts in the pro-
tista but as much as 95% in vertebrates (Cavalier-Smith, 1985).
As far as human genes are concerned, introns are the rule rather than the excep-
tion (Chapter 1, section 1.2.1). Some human genes have nevertheless been found
which lack introns. These include the sex determining (
SRY
; Yp11.3) gene
(O'Neil
et al
., 1998), the POU domain transcription factor
POU3F2
(6q16) gene
(Atanasoski
et al
., 1995), the thrombomodulin (
THBD
; 20p11.2) gene (Jackman
et
al
., 1987), the
2-adrenergic receptor (
ADRB2
; 5q32-q34) gene (Kobilka
et al
.,
1987), the
JUN
proto-oncogene (1p31-p32; Hattori
et al
., 1988), the recombina-
tion-activating genes
RAG1
and
RAG2
(11p13), the arylamine
N
-acetyltrans-
ferase (
AAC1
; 8p21.3-p23.1) gene (Grant
et al
., 1989), the 13 genes of the
