264
W.-H. U et al.
respectively. Positions 230 and 309 are considerably less important because according to Merbs and Nathans (1993), they cause shifts of 1 nm or less, though Asenjo
et al. (1994) obtained estimates of 0--4 and 0-3 nm, respectively. The same comment applies to position 233 because Shyue et al. (1998) estimated a shift of -1 nm
for Gly233Ser and Merbs and Nathans (1993) estimated a shift of -0.4 nm for
Ala233Ser, though Asenjo et al. (1994) gave an estimate of -0-4 for Ala233Ser.
The significance of position 116 is not certain. Shyue et al. (1998) found no effect
of changes at this position, whereas Asenjo et al. (1994) found some effect when
the change Tyr116Ser was introduced into a red (long wavelength) pigment. All
other differences among the available primate X-linked opsin sequences do not
appear to have any discernible effects on spectral tuning.
The above residues are those that are involved in the spectral tuning of X-linked
pigments in higher primates. Sites 185, 277, and 285 have also been found to be
major critical sites in the spectral tuning of red-green vision in other vertebrates
(Yokoyama and Yokoyama 1990; Yokoyama and Radlwimmer 1998). Additional
critical residues have been found in other mammals. For example, the changes
His197Tyr and Ala308Ser were estimated to cause 28 and 18 nm shifts in the
mouse, respectively (Sun et al. 1997). For more details, see Yokoyama and
Radlwimmer (1998).
5 Frequent Gene Conversion Between
X-linked Opsin Alleles or Genes
The tight linkage and high similarity (-98%) between the red and green opsin
genes in Old World primates provide a favorable condition for gene conversion to
occur between them. This possibility was first noted for exons 4 and 5 between the
two genes in OWMs (Balding et al. 1992; Ibbotson et al. 1992) and for exon 3
between the two genes in humans (Winderickx et al. 1993). Deeb et al. (1994) and
Reyniers et al. (1995) provided further evidence of frequent gene conversion between the two genes in humans and OWMs.
A remarkable example of gene conversion was found when introns 4 of human
red and green opsin genes were sequenced (Shyue et al. 1994). The two introns
were identical, though a > 8% divergence was expected between them because
human red and green opsin genes arose from a duplication before the divergence of
the OWM and human lineages, i.e., more than 25 million years ago.
The complete human red opsin gene and a large region of the human green
opsin gene have been sequenced at the Sanger Center, Cambridge, UK, and a comparison reveals that all noncoding parts that are now available for comparison (the
3' part of intron 3, intron 4, intron 5 and the 3' flanking region) are identical or
almost identical between the two genes (Table 1), suggesting frequent gene conversion between these two genes (Zhao et al. 1998). In contrast, the divergences in
exons 4 and 5 are 3.8% and 4.7% (Table 1). It is likely that gene conversion has
also occurred in exons. In fact, exons 1 and 6 of the red and green opsin genes have
W.-H. U et al.
respectively. Positions 230 and 309 are considerably less important because according to Merbs and Nathans (1993), they cause shifts of 1 nm or less, though Asenjo
et al. (1994) obtained estimates of 0--4 and 0-3 nm, respectively. The same comment applies to position 233 because Shyue et al. (1998) estimated a shift of -1 nm
for Gly233Ser and Merbs and Nathans (1993) estimated a shift of -0.4 nm for
Ala233Ser, though Asenjo et al. (1994) gave an estimate of -0-4 for Ala233Ser.
The significance of position 116 is not certain. Shyue et al. (1998) found no effect
of changes at this position, whereas Asenjo et al. (1994) found some effect when
the change Tyr116Ser was introduced into a red (long wavelength) pigment. All
other differences among the available primate X-linked opsin sequences do not
appear to have any discernible effects on spectral tuning.
The above residues are those that are involved in the spectral tuning of X-linked
pigments in higher primates. Sites 185, 277, and 285 have also been found to be
major critical sites in the spectral tuning of red-green vision in other vertebrates
(Yokoyama and Yokoyama 1990; Yokoyama and Radlwimmer 1998). Additional
critical residues have been found in other mammals. For example, the changes
His197Tyr and Ala308Ser were estimated to cause 28 and 18 nm shifts in the
mouse, respectively (Sun et al. 1997). For more details, see Yokoyama and
Radlwimmer (1998).
5 Frequent Gene Conversion Between
X-linked Opsin Alleles or Genes
The tight linkage and high similarity (-98%) between the red and green opsin
genes in Old World primates provide a favorable condition for gene conversion to
occur between them. This possibility was first noted for exons 4 and 5 between the
two genes in OWMs (Balding et al. 1992; Ibbotson et al. 1992) and for exon 3
between the two genes in humans (Winderickx et al. 1993). Deeb et al. (1994) and
Reyniers et al. (1995) provided further evidence of frequent gene conversion between the two genes in humans and OWMs.
A remarkable example of gene conversion was found when introns 4 of human
red and green opsin genes were sequenced (Shyue et al. 1994). The two introns
were identical, though a > 8% divergence was expected between them because
human red and green opsin genes arose from a duplication before the divergence of
the OWM and human lineages, i.e., more than 25 million years ago.
The complete human red opsin gene and a large region of the human green
opsin gene have been sequenced at the Sanger Center, Cambridge, UK, and a comparison reveals that all noncoding parts that are now available for comparison (the
3' part of intron 3, intron 4, intron 5 and the 3' flanking region) are identical or
almost identical between the two genes (Table 1), suggesting frequent gene conversion between these two genes (Zhao et al. 1998). In contrast, the divergences in
exons 4 and 5 are 3.8% and 4.7% (Table 1). It is likely that gene conversion has
also occurred in exons. In fact, exons 1 and 6 of the red and green opsin genes have