Worm Breeder's Gazette 15(3): 29 (June 1, 1998)

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.

MAB-3 is a direct transcriptional repressor of vitellogenin transcription

Woelsung Yi, David Zarkower

Department of Biochemistry and Institute of Human Genetics, University of Minnesota Medical School, Minneapolis, MN 55455

        Nematodes, insects, and possibly mammals control sexual
development using a class of transcription factors containing a newly
defined DNA binding motif called a DM domain.  In worms, we recently
found that mab-3 encodes a protein with two DM domains, while in flies,
the doublesex (dsx) gene has one DM domain (1).  Surprisingly, despite
the different structures of the proteins, we have found that the
preferred in vitro DNA binding sites of MAB-3 and DSX are overlapping. 
We found potential MAB-3 binding sequences in the proximal promoters of
three of the six vitellogenin genes, vit-1, vit-2 and vit-6.  The vit-2
and vit-6 sites are completely conserved between C. elegans and C.
briggsae.  If other vit promoters have MAB-3 binding sites, they may be
less proximal to the transcriptional start sites.   
        A short (247 bp) region of the vit-2 promoter is sufficient for
sex-, tissue- and stage-specific transcription of vit-2 (adult
hermaphrodite intestine).  We found that a GFP reporter with this
promoter is expressed in mab-3 mutant males.  MAB-3 binds this promoter
region in vitro, and mutating the MAB-3 binding sequence eliminates
binding. We tested the in vivo relevance of the MAB-3 DNA binding site
by mutating the GFP reporter.  Mutations that disrupt binding of MAB-3
to the vit-2 promoter in vitro eliminate sex-specific regulation in vivo
without affecting tissue- or stage-specificity.  This strongly suggests
that MAB-3 is a direct transcriptional repressor of vitellogenins in the
male intestine.  Strikingly, the site bound by MAB-3 in vit-2 and the
sites bound by DSX in the Drosophila YP1  (yolk protein 1) promoter are
very similar, despite the fact that the vitellogenins and the YPs
themselves are unrelated at the protein sequence level.  Thus it appears
in this case that the regulators and their regulatory sites are much
more highly conserved than the structural proteins they control.  
        What regulates mab-3?  mab-3 mRNA levels are controlled by
TRA-1, a zinc finger putative transcription factor, but this regulation
might be direct or indirect.  The mab-3 promoter directs male-specific
transcription in the intestine and in various male sensory neurons,
including the V ray neurons, a hook neuron, two spicule neurons, and the
ASH cells.  Two positively acting promoter elements are required for
this expression, one for intestine and the other for neurons.  While
mab-3 is transcriptionally regulated by    tra-1, no TRA-1 DNA binding
sites seem to be required for regulation of the mab-3 promoter, and thus
we conclude that regulation by tra-1 is probably indirect.  We hope to
find the direct regulators by screening for mutations that cause
inappropriate expression of the mab-3::GFP reporter.   

1.  Raymond et al. (1998) Nature 391:691-695.