Worm Breeder's Gazette 14(1): 60 (October 1, 1995)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Dept. of Biology, MIT, Cambridge, MA 02139
The paired domain was first found in the Drosophila segmentation gene paired and subsequently in four other fly genes (gsb, gsbn, poxn, poxm) and in vertebrates. Paired domains are ~128 amino acid residues in length and bind DNA with sequence specificity. Some paired domain proteins also contain a homeodomain and an octapeptide motif. Mice and humans each have nine known paired-domain containing (Pax) genes. The Pax genes have been classified into six classes (I-VI in alignment) based on the sequence of the paired domain and the presence of other motifs. Pax genes all appear to play roles in early development. Mutations in three of the nine human genes (PAX2, PAX3 and PAX6) are associated with human genetic disorders, and overexpression of PAX genes has been implicated in certain cancers, such as the rhabdomyosarcomas. Previously we showed that vab-3 mutations affect a Pax-6 homolog of C. elegans (Ref.1). We have sought other Pax genes in worms. We used degenerate primers complementary to the conserved regions GIRPCDI and MFAWEIR of the paired box (see alignment) in PCR experiments using worm genomic DNA as a template and thus isolated PCR products corresponding to three Pax genes. We did not isolate products corresponding to vab-3 in these experiments. We probed YAC grids with these PCR products and found that two of the new Pax genes mapped to the cluster of LGIV (temporarily dubbed pax-a and pax-b), while the third (pax-c) mapped to LGV. We have isolated genomic DNA for the pax genes on LGIV and a cDNA for pax-a. pax-c appears to lie in a cosmid gap, and we have only partial sequence of the paired domain for this gene. pax-a lies just to the left of dpy-13. pax-a appears to be a member of the Pax-2/5/8 subclass (78% identity to PAX2 in the paired domain; see alignment for details). pax-a also contains the characteristic octapeptide C-terminal to the paired domain but unlike Pax-2/5/8 does not contain anything resembling a partial homeodomain. This organisation is more similar to the fly gene poxn, which contains only a paired domain and an octapeptide. A GFP reporter construct for pax-a was expressed in neurons in the head and tail. paxb lies in the cluster of LGIV. From partial sequence of its paired domain (not shown), pax-b is most similar to that of pax-a, and it also encodes the characteristic octapeptide, suggesting that these two genes arose by duplication and divergence. The genome sequencing project recently found a paired-domain containing gene in the cluster of LGII. The sequence of this predicted gene (F27E5.2) is most similar to the Pax-3/7/gsb class, in that it encodes a class-II paired domain, an octapeptide and a homeodomain. The paired domain is 58% identical to those of mammalian Pax-3 and Pax-7 and is highly divergent in the C-terminal part of the paired domain. F27E5.2-GFP reporter constructs are expressed strongly in ventral hypodermal cells during embryogenesis, particularly during morphogenesis. We are examining existing mutants mapping to the pax genomic regions as candidates for being pax mutants. In addition, in collaboration with Ron Plasterk's laboratory we are screening for Tc1 insertions near these genes with the aim of disrupting them and doing genetics. Noll proposed that four types of Pax gene existed prior to the divergence of protostomes and deuterostomes, i.e., in an ancestral metazoan (Ref.2). Our results tentatively support this hypothesis. In this view, one ancestral Pax gene evolved to give rise to Pax-3/7 in vertebrates, prd/gsb/gsbn in Drosophila, and F27E5.2 in C. elegans. A second gene was ancestral to Pax-2/5/8 in vertebrates, poxn in flies and perhaps pax-a/b in worms. A third gene was ancestral to Pax-6 (and possibly Pax-4) in vertebrates, eyeless in flies and vab-3/mab-18 in worms. The fourth gene gave rise to Pax-1/9 in vertebrates and poxm in flies. As yet we cannot tell if pax-c V is a member of this fourth class, which from these evolutionary considerations should exist somewhere in the worm genome. 1. Chisholm and Horvitz, Nature, in press. 2. Noll, Curr. Opin. Gen. Dev. 3: 595 (1993).