NOVEL UBIQUITIN-LIKE RIBOSOMAL-PROTEIN FUSION GENES FROM THE NEMATODES CAENORHABDITIS-ELEGANS AND CAENORHABDITIS-BRIGGSAE

Among eukaryotes studied to date, homologs of the yeast 76-amino acid ribosomal protein have invariably been found to be cotranslated with u biquitin. However, in the nematodes Caenorhabditis elegans and Caenorh abditis briggsae, a 70-amino acid domain with only 40% identity to ubi quitin is cotranslated with a homolog of the ribosomal protein. In the nematode ubiquitin-like (UbL) proteins, the nucleotide sequence of th e UbL coding region is 92% identical in C. elegans and C. briggsae. Th e corresponding gene sequence contains a single intron at a location i dentical to that found in the polyubiquitin gene of C. elegans, furthe r confirming that the ubl genes are evolutionarily related to ubiquiti n. The ribosomal protein portion of the UbL polypeptide consists of 93 amino acids and is 68% identical to the human homolog. The ribosomal protein portion of UbL is longer than in other homologs, with the addi tional sequence being present as a basic carboxyl extension. The ubl g ene is constitutively expressed in all life cycle stages of C. elegans . A comparison of the nematode UbL sequences with other ubiquitin-like genes reveals a pattern of sequence conservation, which suggests that the ubiquitin-like proteins may have conserved functional domains.