The organization of genes into operons, clusters of genes that are 
co-transcribed to produce polycistronic pre-mRNAs, is a trait found in a wide 
range of eukaryotic groups, including multiple animal phyla. Operons are present 
in the class Chromadorea, one of the two main nematode classes, but their 
distribution in the other class, the Enoplea, is not known. We have surveyed the 
genomes of Trichinella spiralis, Trichuris muris, and Romanomermis culicivorax 
and identified the first putative operons in members of the Enoplea. Consistent 
with the mechanism of polycistronic RNA resolution in other nematodes, the mRNAs 
produced by genes downstream of the first gene in the T. spiralis and T. muris 
operons are trans-spliced to spliced leader RNAs, and we are able to detect 
polycistronic RNAs derived from these operons. Importantly, a putative 
intercistronic region from one of these potential enoplean operons confers 
polycistronic processing activity when expressed as part of a chimeric operon in 
Caenorhabditis elegans. We find that T. spiralis genes located in operons have 
an increased likelihood of having operonic C. elegans homologs. However, operon 
structure in terms of synteny and gene content is not tightly conserved between 
the two taxa, consistent with models of operon evolution. We have nevertheless 
identified putative operons conserved between Enoplea and Chromadorea. Our data 
suggest that operons and "spliced leader" (SL) trans-splicing predate the 
radiation of the nematode phylum, an inference which is supported by the 
phylogenetic profile of proteins known to be involved in nematode SL 
trans-splicing.