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The nematode, Caenorhabdltls e l egans , conta ins a family of s ix genes that code for v i t e l l o g e n l n s . Here we repor t the complete nucleot lde sequence of one of these genes, v i t 5 . The gene spec i f i e s a mRNA of 4869 nuc l eo t ide s , including un t rans la ted regions of 9 bases at the 5 ' end and 51 bases at the 3 ' end. Vit-5 contains four short in t rons t o t a l l i n g 218 bp. The predicted v i t e l l o g e n l n , ypl70A, has a molecular weight of 186,430. At i t s N terminus i t i s c l e a r l y r e l a t ed to the v i t e l l o g e n i n s of v e r t e b r a t e s . However, the vl t-5-encoded prote in does not contain a s e r i n e r i c h sequence r e l a t ed to the ve r t eb ra t e v l t e l l i n , phosv i t in . In f a c t , the amino acid conposit ion of the nematode pro te in i s very s imi la r to that of the ve r t eb ra t e prote in without phosv i t in . Vi t -5 has a highly asymmetric codon choice d i c t i ona ry . The favored codons are d i f fe ren t from those favored in other organisms, but a re c h a r a c t e r i s t i c of highly expressed C. elegans genes. The strong s e l e c t i o n agains t ra re codons i s not as grea t near the 5 ' end of the gene; r a re codons are 15 times more frequent within the f i r s t 54 bp than in the next 4 .8 kb. INTRODUCTION Four distinct yolk proteins have been identified in the nematode, Caenorhabditls elegans (1). Two of these, ypl70A and ypl70B, are translated as polypeptides estimated to be 170,000 daltons (2). These vitellogenins undergo little If any processing before they are packaged into yolk platelets. In contrast, the two smaller yolk proteins, ypll5 and yp88, are cut from a single precursor polypeptide, VIT180 (3). ypl70A and B and VIT 180 are synthesized only in the intestine of the adult hermaphrodite worm, from which they are cotranslationally secreted into the body cavity (3, 4). There, VIT180 is cleaved and the yolk proteins are subsequently taken up into developing oocytes. The vitellogenins are encoded by a small family of genes (5, 6). ypl70A is specified by vlt-3, vit-4, and vlt-5. These three genes are at least 95Z homologous to one another at the nucleotide sequence level. Vlt-5 mRNA is much more abundant than that from vlt-3 and vit-4 (5; Cane, unpublished observations). ypl70B Is specified by vlt-2. Vit-1 is a pseudogene, about © IRL Press Limited, Oxford, England. 7129 Nucleic Acids Research 95X identical to vit-2 (7). The vlt-1,2 sub-family is about 70Z homologous to the vit-3,4,5 sub-family. VIT180 is encoded by a very distantly related family member, vlt-6 (6). We have recently reported the nucleotide sequences of the regions surrounding the 5' ends of five of the C. elegans vitellogenin genes (7). Our results Indicated that these genes were surprisingly closely related to the vitellogenin genes of vertebrates. Like the nematode genes, the vertebrate genes are expressed in a tissue of endodennal origin, the liver. Like ypl70A and ypl70B, the vertebrate vitellogenins are transported intact to the oocytes. However, unlike the nematode vitellogenins, the vertebrate proteins are cleaved in the oocyte to form vltellins: phosvitin and the lipovitellins (8). In this paper we report the complete nucleotide sequence of vit-5, the first member of the vertebrate/nematode vitellogenin gene family to be sequenced. Our data indicate that although the nematode gene is closely related to the vertebrate gene near the 5' end, the region encoding the serine-rich vertebrate protein, phosvitin, is not present in vlt-5. RESULTS AND DISCUSSION Characteristics of vit-5 The complete nucleotide sequence of vit-5, along with 261 bp of 5' flanking and 27 bp of 3' flanking DNA, is shown in Fig. 1. The location of the 5' end of the mSMA was determined by primer extension using a synthesized 15 bp oligonucleotide primer, accompanied by dideoxy-sequencing. A single, unequivocal extension product was found (data not shown). Similar experiments were performed with vit-2, vlt-4, and vit-6. In each case a single extension product, the same distance 3' from a canonical TATA box sequence was found. The 3 end of the gene was located by sequencing a 3' terminal cDNA clone containing a 50 base poly(A) stretch. The gene contains four short lntrons, totaling 218 bp. The presence of the two introns near the 3' end of the gene was directly demonstrated by sequencing cDNA clones which overlap this region. The presence of the other two introns is Inferred: they contain canonical splice sites at their 5' and 3 boundaries (Table 1); they contain stop codons in all three reading frames; and they are at least 70Z A + T. Excision of these proposed introns at the positions indicated allows translation utilizing primarily the highly favored codons typical of the £. elegans vitellogenin genes (see below). We presume that translation begins at the first AUG in each mRNA. This assumption is supported by: 1) The presence of a highly hydrophobic amlno