Worm Breeder's Gazette 11(5): 42

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.

pal-1, a Homeobox Gene that Regulates a Homeobox Gene

David Waring and Cynthia Kenyon

pal-1 is a gene required for the formation of rays by the V6 cell in 
males.  As we have described previously, pal-1 effectively prevents 
the T cell from inducing V6 to generate alae and, thereby, allows V6 
to generate rays.  In the absence of the T cell (following laser 
ablation), pal-1 is not required for V6 ray formation.
Previously we presented evidence that pal-1 positively activates mab-
5.  Supporting this model we have found that pal-1 is suppressed by 
the mab-5 gain of function allele e1751 (formerly lin-21).  In unc-79(
e1061) 91) 51); 
90) males, only 5/100 sides had fewer than the wild-
type number of rays in the fan; and in those cases, it appeared that 
V6 generated a few rays but not the wild-type number (V6 makes no rays 
at all on 85% of the sides in pal-1(e2091); 90)).  
The e1751 lesion is a duplication of a region of DNA containing mab-5.  
As Salser and Kenyon describe in an abstract in this gazette, it seems 
likely that the e1751 phenotype is the result of inappropriate 
transcription of mab-5.  The fact that pal1+ activity is not required 
in the presence of the e1751 mutation suggests that pal-1 turns on mab-
5 at the level of transcription.  Whether or not it does so directly 
is unknown.
At last report we had narrowed the search for pal-1 to a 14 kb clone 
that carried the homeobox ceh-3.  Now we have a 5.5 kb, ceh-3 
containing clone, which rescues the pal-1 phenotype completely.  We 
have found 2 independent cDNA clones which are each 1.3 kb (from Chris 
Martin's miracle library).  One is just a few bases longer than the 
other.  The 5.5 kb clone that rescues pal-1 contains about 1 kb 
upstream of the 5' end of the cDNAs and about 400 basepairs downstream 
of the 3' end of the cDNAs.  For this reason we believe that ceh-3 is 
pal-1 (although we have not yet ruled out the alternative that there 
is a second gene on this clone).
These cDNAs encode a ~250aa protein that contains the ceh-3 
homeodomain.  As Burglin and Ruvkin showed, this homeodomain is 
homologous to the caudal gene of Drosophila.  Both cDNAs have part of 
what appears to be a trans-spliced leader sequence (the longer has 10 
bases homologous to the splice-leader), so we believe these cDNAs are 
virtually full length.  We have found no other conserved domains of 
any known significance, but we have found one curious homology to the 
mab-5 gene.  In the mab-5 cloning paper, Costa et al.  pointed out a 
serine rich region followed by a poly-alanine stretch.  Interestingly, 
pal-1 has a very similar region, which includes an identical stretch 
of 9 amino acids SAAAAAAAN (the San Box).  Such poly-amino acid 
stretches are common in homeodomain proteins, but their function is 
unknown.  While this homology may be insignificant, the fact that both 
pal-1 and mab-5 act in V6 to promote ray formation makes this homology 
mildly but not profoundly tantalizing.
Finally, to determine if pal-1 acts directly on mab-5 we are looking 
for pal-1 binding sites in the vicinity of mab-5.  (In principle, if 
pal-1 binds we have made a pal-1/glutatione-S-transferase fusion 
protein (glu-pal) and expressed it in E.  coli.  Our results are very 
preliminary; but we have been able to purify this fusion protein, and 
we have seen sequence specific binding (as judged by band shifts) to 
an oligo-nucleotide probe known to be bound in vitro by engrailed and 
several other homeodomain proteins (E.  Martin-Blanco and T Kornberg, 
personal communication).  We hope to be able to use the glu-pal fusion 
to identify authentic pal-1 binding sites near mab-5.