Biology Reference
In-Depth Information
32-aa repeat
exons
Rest of
the gene
Intron loss
Ancestral class 2
aminoacyl-tRNA
synthetase gene
Prolyl-tRNA synthetase gene
Inverted duplication
and translocation
HO3
Histidyl-tRNA
synthetase gene
Human
Histidyl-tRNA synthetase gene
Fugu
Translocation through
the first intron
Histidyl-tRNA
synthetase gene
Class 2 aminoacyl-tRNA
synthetase gene
Translocation
through the
second intron
Intron
loss
Tryptophanyl-tRNA
synthetase gene
Tryptophanyl-tRNA
synthetase gene
Figure 9.2.
Hypothetical translocation events in the evolution of the aminoacyl-tRNA
synthetase genes (after Brenner and Corrochano, 1996). The two exons encoding the 32-
amino acid repeat and the rest of the gene are shown by stippled and hatched boxes
respectively. An ancestral class 2 aminoacyl-tRNA synthetase gene containing the two
repeat-containing exons gave rise to the prolyl-tRNA synthetase and histidyl-tRNA
synthetase (
HARS
) genes. The prolyl-tRNA synthetase gene of extant animals contains
several copies of the repeat and is fused to the gene encoding glutamyl-tRNA synthetase
(
EPRS
). The
HARS
gene underwent an inverted duplication and a translocation in the
human lineage resulting in two genes in opposite orientation. The translocation may
have occurred through the first intron, resulting in the capture of a new exon in the
histidyl-tRNA synthetase-homologous (HO3) gene. In the
Fugu
lineage, a translocation
through the first intron also allowed the capture of a new exon. The human tryptophanyl-
tRNA synthetase (
WARS
) gene, encoding a class 1 enzyme, has captured the two exon
repeat by a translocation into a class 2 gene. The loss of a further intron resulted in the
structure of the extant
WARS
gene in humans.
duplication and divergence some 750 Myrs ago whilst
PAH
and
TH
diverged sub-
sequently about 600 Myrs ago (Craig
et al
., 1986; Ledley
et al
., 1987;
Figure 9.3
).
The synteny of the extant
TPH
and
TH
genes implies that the
PAH
gene must
have been translocated to chromosome 12 only after the second duplicational
event 600 Myrs ago (
Figure 9.3
). A similar explanation may pertain for the insulin
(
INS
; 11p15.5), insulin-like growth factor 1 (
IGF1
; 12q22-q24.1) and insulin-like
growth factor 2 (
IGF2
; 11p15.5) genes (Bell
et al
., 1985; Tricoli
et al
., 1984).
In humans and orangutans, two tRNA
Asn
gene clusters are located on the short
and long arms of chromosome 1, respectively (
TRN
, 1p36.1;
TRNL
, 1q21)
(Buckland
et al
., 1992). By contrast, Old World monkeys possess only one tRNA
Asn
gene cluster on chromosome 1p whilst the capuchin (a New World monkey)
