The nuclear receptor superfamily has undergone extensive proliferation anddiversification in nematodes

The nuclear receptor (NR) superfamily is the most abundant class of transcr iptional regulators encoded in the Caenorhabditis elegans genome, with >200 predicted genes revealed by the screens and analysis of genomic sequence r eported here. This is the largest number of NR genes yet described from a s ingle species, although our analysis of available genomic sequence from the related nematode Caenorhabditis briggsae indicates that it also has a larg e number. Existing data demonstrate expression for 25% of the C. elegans NR sequences. Sequence conservation and statistical arguments suggest that th e majority represent functional genes. An analysis of these genes based on the DNA-binding domain motif revealed that several NR classes conserved in both vertebrates and insects are also represented among the nematode genes, consistent with the existence of ancient NR classes shared among most, and perhaps all, metazoans. Mon of the nematode NR sequences, however, are dis tinct from those currently known in other phyla, and reveal a previously un observed diversity within the NR superfamily. In C, elegans, extensive prol iferation and diversification of NR sequences have occurred on chromosome V , accounting for > 50% of the predicted NR genes.