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Mol. Cell. Biol., 08 1995, 4430-4440, Vol 15, No. 8
G Deshpande, J Stukey and P Schedl
The primary sex determination signal, the X chromosome-to-autosome (X/A)
ratio, controls the choice of sexual identity in the Drosophila
melanogaster embryo by regulating the activity of the early promoter of the
Sex-lethal gene, Sxl-Pe. This promoter is activated in females (2X/2A),
while it remains off in males (1X/2A). Promoter activation in females is
dependent upon X-linked numerator genes. One of these genes, sisterless-b
(sis-b), corresponds to the scute (sc) locus of the achaete-scute complex,
and it encodes a helix-loop-helix transcription factor. In the studies
reported here we have used monoclonal antibodies to study the expression
and functioning of the sc(sis-b) protein. Sc is first detected at nuclear
cycle 6 to 7, well before Sxl-Pe is first active. At this stage, the
protein is in the cytoplasm, not the nucleus. Only after the formation of
the syncytial blastoderm, at nuclear cycle 10 to 11, does a substantial
fraction of the protein enter the nucleus, and this nuclear import closely
coincides with the initial activation of Sxl-Pe. Consistent with the idea
that the dose of sc(sis-b) is critical for its function as an X-chromosome
counting element, wild-type syncytial blastoderm embryos could be grouped
into two classes based on the level of protein. Western blot (immunoblot)
analysis demonstrates that this difference in protein level correlates
directly with the activity state of the Sxl gene. Finally, we provide the
first direct evidence that Sc forms heteromeric complexes in vivo in early
embryos with the maternally derived helix-loop-helix protein Daughterless.
This in vivo complex is likely to be critical for Sc function in Sxl-Pe
activation.
Copyright © 1995, American Society for Microbiology
scute (sis-b) function in Drosophila sex determination
Department of Molecular Biology, Princeton University, New Jersey 08544, USA.
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