Nematodes are the only group of organisms in which both cis- and trans-splicing of nuclear mRNAs are known to occur. Most Caenorhabditis elegans introns are exceptionally short, often only 50 bases long. The consensus donor and acceptor splice site sequences found in other animals are used for both cis- and trans-splicing. In order to identify the machinery required for these splicing events, we have characterized the C. elegans snRNAs. They are similar in sequence and structure to those characterized in other organisms, and several sequence variations discovered in the nematode snRNAs provide support for previously proposed structure models. The C. elegans snRNAs are encoded by gene families. We report here the sequences of many of these genes. We find a highly conserved sequence, the proximal sequence element (PSE), about 65 bp upstream of all 21 snRNA genes thus far sequenced, including the SL RNA genes, which specify the snRNAs that provide the 5' exons in trans-splicing. The sequence of the C. elegans PSE is distinct from PSE's from other organisms.
Caenorhabditis elegans vitellogenins are encoded by a family of six genes, one of which, vit-5, has been previously sequenced and shown to be surprisingly closely related to the vertebrate vitellogenin genes. Here we report an alignment of the amino acid sequences of vitellogenins from frog and chicken with those from three C. elegans genes: vit-5 and two newly sequenced genes, vit-2 and vit-6. The four introns of vit-6 are all in different places from the four introns of vit-5, but three of these eight positions are identical or close to intron locations in the vertebrate vitellogenin genes. The encoded polypeptides have diverged from one another sufficiently to allow us to draw some conclusions about conserved positions. Many cysteine residues have been conserved, suggesting that vitellogenin structure has been maintained over a long evolutionary distance and is dependent upon disulfide bonds. In addition, a 20-residue segment shows conservation between the vertebrate and the nematode vitellogenins. This sequence may play a highly conserved role in vitellogenesis, such as specific recognition by oocytes. On the whole, however, selection may be acting more strongly on amino acid composition and codon usage than on amino acid sequence, as might be expected for abundant storage proteins: The amino acid compositions of vit-2, vit-5, and vit-6 products are remarkably similar, despite the fact that the sequence of the vit-2 protein is only 22% and 50% identical to the sequences of vit-6 and vit-5 proteins, respectively.
The nematode, Caenorhabditis elegans, has a six-member gene family encoding vitellogenins, the yolk protein precursors. These genes are expressed exclusively in the intestine of the adult hermaphrodite. Here we report the cloning of all five members of the homologous gene family from another Caenorhabditis species, Caenorhabditis briggsae. Nucleotide sequence analysis of these genes reveals they are about 85% identical to the C. elegans genes in the coding regions. Overall similarity is much reduced in noncoding and flanking regions. However, two repeated heptamers, previously identified in the upstream regions of the C. elegans genes, are largely conserved in both location and sequence in C. briggsae. Conservation of certain of these heptamers suggests that proteins bound at these positions may be especially important to promoter function and/or regulation. Comparative sequence analysis also suggests the possibility that the first 70 bases of the vitellogenin mRNAs can be folded into stable secondary structures. Almost all base differences between the two species occur in sequences predicted to be unpaired, suggesting that the ability to form intrastrand base pairs has been selected during Caenorhabditis evolution.
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