Interpretation of X Chromosome Dose at Sex-lethalRequires Non-E-Box Sites for the Basic Helix-Loop-Helix Proteins SISB and Daughterless

Abstract

For Drosophila melanogaster flies, sexual fate is determined by the X chromosome number. The basic helix-loop-helix protein product of the X-linked sisterlessB(sisB or scute) gene is a key indicator of the X dose and functions 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 activate transcription. We examined SISB-Da binding atSxl by using footprinting and gel mobility shift assays and found that SISB-Da binds numerous clustered sites in the establishment promoter Sxl_Pe . Surprisingly, most SISB-Da sites at Sxl_Pe differ from the canonical CANNTG E-box motif. These noncanonical sites have 6-bp CA(G/C)CCG and 7-bp CA(G/C)CTTG cores and exhibit a range of binding affinities. We show that the noncanonical sites can mediate SISB-Da-activated transcription in cell culture. P-element transformation experiments show that these noncanonical sites are essential for Sxl_Pe activity in embryos. Together with previous deletion analysis, the data suggest that the number, affinity, and position of SISB-Da sites may all be important for the operation of the Sxl_Pe switch. Comparisons with other dose-sensitive promoters suggest that threshold responses to diverse biological signals have common molecular mechanisms, with important variations tailored to suit particular functional requirements.

ACKNOWLEDGMENTS

We thank Paul Schedl, Dan Kalderon, Jym Mohler, and Tulle Hazelrigg for thoughtful advice during the course of this work and Larry Chasin for valuable assistance with cell culture. L. Chasin, R. Prywes, J. Manley, J. Posakony, and M. Caudy generously provided plasmids and reagents. We are grateful to Teresa Lamb, P. Schedl, and Dan Kalderon for help with the manuscript.

This research was funded by a Searle Community Trust Award and American Cancer Society grant RPG-97-079-01-DB to J.W.E. and by an NIH grant to Paul Schedl (Princeton University).