D. Yang et al., Interpretation of X chromosome dose at Sex-lethal requires non-E-box sitesfor the basic helix-loop-helix proteins SISB and daughterless, MOL CELL B, 21(5), 2001, pp. 1581-1592
For Drosophila melanogaster flies, sexual fate is determined by the X chrom
osome number. The basic helix-loop-helix. protein product of the X-Linked s
isterlessB (sisB or scute) gene is a key indicator of the X dose and functi
ons to activate the switch gene Sex-lethal (Sxl) in female (XX), but not in
male (XY), embryos. Zygotically expressed sisB and maternal daughterless (
da) proteins are known to form heterodimers that bind E-box sites and activ
ate transcription. We examined SISB-Da binding at Sri by using footprinting
and gel mobility shift assays and found that SISB-Da binds numerous cluste
red sites in the establishment promoter Sxl(Pe). Surprisingly, most SISB-Da
sites at Sxl(Pe) differ from the canonical CANNTG E-box motif. These nonca
nonical sites have 6-bp CA(GIC)CCG and 7-bp CA(GIC)CTTG cores and exhibit a
range of binding affinities. We show that the noncanonical sites can media
te SISB-Da-activated transcription in cell culture. P-element transformatio
n experiments show that these noncanonical sites are essential for Sxl(Pe)
activity in embryos. Together with previous deletion analysis, the data sug
gest that the number, affinity, and position of SISB-Da sites may all be im
portant for the operation of the Sxl(Pe) switch. Comparisons with other dos
e-sensitive promoters suggest that threshold responses to diverse biologica
l signals have common molecular mechanisms, with important variations tailo
red to suit particular functional requirements.