We have cloned the cDNA coding for the Caenorhabditis elegans homologue of RAD51 (CeRAD51) in order to study its regulation in a well characterized metazoan model system as well as to identify possible species-specific features of regulation of recombination. The CeRAD51 gene is located on chromosome IV on the left of the
mec-3 gene. The entire genomic locus is composed of 8 exons and 7 introns. By transcriptional analysis and cDNA cloning we identified two alternative mRNAs, both of them including the entire coding region conserved throughout evolution. The shorter transcript does not contain the first two exons and presents the trans-spliced leader SL1 at the 5' end. Because of the mechanics of trans-splicing, it is likely that the two transcripts of CeRAD51 use different promoters. The transcription of the shorter mRNA should begin within the second intron so that, in absence of the donor site, SL1 can trans-splice without competition with cis- splicing. The longer hypothetical protein, that we call CeRad51L, differs from the shorter one, CeRad51S, solely for the first 38 aminoacids that are missing in the latter. CeRad51S approximates the size of most eukaryotic Rad51/Dmc1 proteins, while the only other protein of the group, beside CeRad51L with a long aminoterminal peptide is the budding yeast Rad51 protein. We are now trying to assign a role to the two anternative products. Furthermore, since the RAD51 monomers are known to form in yeast long concatamers to perform the strand transfer activity, we are studying, by "two hybrid" analysis, the protein domains of CeRad51 involved in this interaction.