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.

Pax in worms

Andrew Chisholm, Bob Horvitz

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
        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
        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).