X:A ratio, the primary sex-determining signal in Drosophila, is transduced by helix-loop-helix proteins.
Drosophila determines its sex by "counting" X chromosomes. We show that premature expression of the pair-rule segmentation gene hairy interferes with this process, resulting in female-specific lethality by inhibiting initiation of the master control gene Sex-lethal (Sxl). The female-specific lethality can be suppressed by a constitutive Sxl allele or by extra copies of X-linked "counting elements." These results are best explained by competition between hairy and other helix-loop-helix transcription factors that act in chromosome counting. We have confirmed this model by showing that misexpression of the achaete-scute T4 gene induces ectopic Sxl expression and male-specific lethality, confirming that achaete-scute T4 is the sisterless-b counting element. We propose that X chromosomes are counted through heterodimers of helix-loop-helix transcription factors that act synergistically to initiate Sxl expression.
Parkhurst
S M
SM
Zoology Department, University of Oxford, England.
Bopp
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Ish-Horowicz
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D000818:Animals; D001756:Blastoderm / Q000502:physiology; D003433:Crosses, Genetic; D004268:DNA-Binding Proteins / Q000378:metabolism*; D004331:Drosophila melanogaster / Q000196:embryology / Q000235:genetics*; D004625:Embryo, Nonmammalian / Q000502:physiology; D005260:Female; D005804:Genes, Lethal; D008297:Male; D015183:Restriction Mapping; D012732:Sex Determination Analysis; D014161:Transduction, Genetic; D014170:Transformation, Genetic; D014960:X Chromosome
D004268:DNA-Binding Proteins