The RAD6/UBC2 gene from Saccharomyces cerevisiae encodes a ubiquitin-conjugating enzyme involved in DNA repair, induced mutagenesis, and sporulation. Here we report the isolation and characterization of the Caenorhabditis elegans RAD6 homolog designated ubc-1. Ubc-1 encodes a 21.5-kD protein that shares considerable identity with RAD6 (66%) as well as with other RAD6 homologs, including Schizosaccharomyces pombe rhp6+ (70%), Drosophila melanogaster Dhr6 (83%), and the two human homologs HHR6A and HHR6B (84% and 83%, respectively). However, UBC-1 is distinct in being the only known RAD6 homolog, other than RAD6 itself, with a carboxy-terminal extension. Analysis of UBC-1 homologs from C. briggsae and Ascaris suum show that the presence of the carboxy-terminal extension is conserved in nematodes. When constitutively expressed from the yeast promoter ADH1, ubc-1 complements the DNA repair functions in a S. cerevisiae rad6 delta mutant. Surprisingly, ubc-1 fails to complement the sporulation function of RAD6, despite its possession of an acidic carboxy-terminal tail. C. elegans UBC-1 is capable of forming a thiol-ester bond with ubiquitin, but, unlike RAD6, is unable to transfer ubiquitin to histone H2B in vitro. Both cis and trans splicing are involved in the maturation of the ubc-1 transcript. The presence of the SL2 trans-splice leader in the ubc-1 transcript suggests that ubc-1 may be co-transcribed as part of a polycistronic message.