; The genomes of most eukaryotes are composed of genes arranged on the chromosomes without regard to function, with each gene transcribed from a promoter at its 5′ end. However, the genome of the free-living nematode; Caenorhabditis elegans; contains numerous polycistronic clusters similar to bacterial operons in which the genes are transcribed sequentially from a single promoter at the 5′ end of the cluster. The resulting polycistronic pre-mRNAs are processed into monocistronic mRNAs by conventional 3′ end formation, cleavage, and polyadenylation, accompanied by trans-splicing with a specialized spliced leader (SL), SL2. To determine whether this mode of gene organization and expression, apparently unique among the animals, occurs in other species, we have investigated genes in a distantly related free-living rhabditid nematode in the genus; Dolichorhabditis; (strain CEW1). We have identified both SL1 and SL2 RNAs in this species. In addition, we have sequenced a; Dolichorhabditis; genomic region containing a gene cluster with all of the characteristics of the; C. elegans; operons. We show that the downstream gene is trans-spliced to SL2. We also present evidence that suggests that these two genes are also clustered in the; C. elegans; and; Caenorhabditis briggsae; genomes. Thus, it appears that the arrangement of genes in operons pre-dates the divergence of the genus; Caenorhabditis; from the other genera in the family Rhabditidae, and may be more widespread than is currently appreciated.;
