Structures of two molluscan hemocyanin genes: Significance for gene evolution

We present here the description of genes coding for molluscan hemocyanins. Two distantly related mollusks, Haliotis tuberculata and Octopus dofleini, were studied. The typical architecture of a molluscan hemocyanin subunit, w hich is a string of seven or eight globular functional units (FUs, designat ed a to h, about 50 kDa each), is reflected by the gene organization: a ser ies of eight structurally related coding regions in Haliotis, corresponding to FU-a to FU-h, with seven highly variable linker introns of 174 to 3,198 bp length tall in phase 1). In Octopus seven coding regions (FU-a to FU-g) are found, separated by phase 1 introns varying in length from 100 bp to 9 10 bp. Both genes exhibit typical signal (export) sequences, and in both ca ses these are interrupted by an additional intron. Each gene also contains an intron between signal peptide and FU-a and in the 3' untranslated region . Of special relevance for evolutionary considerations are introns interrup ting those regions that encode a discrete functional unit. We found that fi ve of the eight FUs in Haliotis each are encoded by a single exon, whereas FU-f, FU-g, and FU-a are encoded by two, three and four exons, respectively . Similarly, in Octopus four of the FUs each correspond to an uninterrupted exon, whereas FU-b, FU-e, and FU-f each contain a single intron. Although the positioning of the introns between FUs is highly conserved in the two m ollusks, the introns within FUs show no relationship either in location nor phase. It is proposed that the introns between FUs were generated as the e ight-unit polypeptide evolved from a monomeric precursor, and that the inte rnal introns have been added later. A hypothesis for evolution of the ring- like quaternary structure of molluscan hemocyanins is presented.