While the nematode Caenorhabditis elegans is more primitive than most egg-laying organisms, it's vitellogenins, or yolk protein precursors, appear to be more complex. C. elegans oocytes accumulate two major classes of yolk proteins. The first consists of two polypeptides with an Mr of about 170,000 (
yp170A and
yp170B) encoded by a family of five closely related genes called
vit-1 through
vit-5. The second class consists of two smaller proteins with Mr values of 115,000 (
yp115) and 88,000 (
yp88) which are cut from a single precursor. Here we report the cloning and analysis of a single-copy gene (
vit-6) that encodes this precursor. The lengths of the gene and its mRNA are about 5 X 10(3) base pairs. Like
vit-1 through
vit-5,
vit-6 is expressed exclusively in adult hermaphrodites. Comparison of portions of the coding sequence indicates that
vit-6 is distantly related to the
vit-1 through
vit-5 gene family. Thus, even though the two classes of yolk proteins are antigenically and physically distinct, they are encoded by a single highly diverged gene family.