Sequence, genomic structure and tissue expression of carp (Cyprinus carpio L.) vertebrate ancient (VA) opsin

We report the isolation and characterisation of a novel opsin cDNA from the retina and pineal of the common carp (Cyprinus carpio L.). When a comparison of the amino acid sequences of salmon vertebrate ancient opsin (sVA) and the novel carp opsin are made, and the carboxyl terminus is omitted, the level of identity between these two opsins is 81% and represents the second example of the VA opsin family. We have therefore termed this C. carpio opsin as carp VA opsin (cVA opsin). We show that members of the VA opsin family may exist in two variants or isoforms based upon the length of the carboxyl terminus and propose that the mechanism of production of the short VA opsin isoform is alternative splicing of intron 4 of the VA opsin gene. The VA opsin gene consists of five exons, with intron 2 significantly shifted in a 3' direction relative to the corresponding intron in rod and cone opsins. The position (or lack) of intron 2 appears to be a diagnostic feature which separates the image forming rod and cone opsin families from the more recently discovered non-visual opsin families (pin-opsins (P), vertebrate ancient (VA), parapinopsin (PP)). Finally, we suggest that lamprey P opsin should be reassigned to the VA opsin family based upon its level of amino acid identity, genomic structure with respect to the position of intron 2 and nucleotide phylogeny.