Purpose: To disclose the structure of visual pigment gene for a protanopia with specific variation.
Methods: Exon 5 fragments of the red and green visual pigment genes from the protanopia with specific variation as well as controls were amplified by polymerase chain reaction (PCR). The PCR products were put through heteroduplex-SSCP analysis and PCR-RFLP (restriction fragment length polymorphism) analysis to clarify the specific variation. The specific variation of the exon 5 DNA fragment from the protanopia was identified by sequencing.
Results: A novel 5'green-3'red hybrid gene fragment without the normal red and green visual pigment gene was discovered in the protanopia. He should only have a single visual pigment gene, 5'green-3'red hybrid gene, on his X chromosome. The fusion point is between codon 285 and codon 296 in exon 5.
Conclusion: Unequal intragenic recombination may occur in exon 5 as well as its upstream. A 5'green-3'red hybrid gene may present independently on the X chromosome without accompanying the red or green visual pigment gene. Single visual pigment gene with 5'green-3'red hybrid fragment of exon 5 could express function which is similar to the green pigment gene. This example also gives in vivo evidence that codons 277 and 285, rather than codon 309, play a major role in tuning the absorption spectra of the red and green visual pigments.