FUNDULUS-HETEROCLITUS VITELLOGENIN - THE DEDUCED PRIMARY STRUCTURE OFA PISCINE PRECURSOR TO NONCRYSTALLINE, LIQUID-PHASE YOLK PROTEIN

We have cloned and sequenced a cDNA encoding a vitellogenin (Vtg) from the mummichog, Fundulus heteroclitus, an estuarine teleost. We constr ucted a liver cDNA library against RNA from estrogen-treated male mumm ichogs. Five overlapping cDNA clones totalling 5,197 bp were isolated through a combination of degenerate oligonucleotide probing of the lib rary and PCR. The cDNA sequence contains a 5,112 bp open reading frame . The predicted primary structure of the deduced 1,704-amino-acid prot ein is 30-40% identical to other documented chordate Vtgs, establishin g this Vtg as a member of the ancient Vtg gene family. Of the previous ly reported chordate Vtg sequences (Xenopus laevis, Gallus domesticus, Ichthyomyzon unicuspis, and Acipenser transmontanus), all four act as precursor proteins to a yolk which is eventually rendered insoluble u nder physiological conditions, either as crystalline platelets or as n oncrystalline granules. The yolk of F. heteroclitus, on the other hand , remains in a soluble state throughout oocyte growth. The putative F. heteroclitus Vtg contains a polyserine region with a relative serine composition that is 10-20% higher than that observed for the other Vtg s. The trinucleotide repeats encoding the characteristic polyserine tr acts of the phosvitin region follow a previously reported trend: TCX c odons on the 5' end and AGY codons toward the 3' end. Whether the diff erence in Vtg primary structure between F. heteroclitus and that of ot her chordates is responsible for the differences in yolk structure rem ains to be elucidated. As the first complete teleost Vtg to be reporte d, these data will aid in designing nucleotide and immunological probe s for detecting Vtg as a reproductive status indicator in F. heterocli tus and other piscine species.