C. elegans sexual fate is established by a chromosome counting mechanism that distinguishes one X chromosome from two. This mechanism assesses the number of X chromosomes relative to the sets of autosomes. Genes encoded on the X chromosome, called X signal elements (XSEs), communicate X chromosome dose by functioning in a cumulative, dose-dependent manner to repress the master switch gene
xol-1.
xol-1 repression in XX embryos results in hermaphrodite development and activation of dosage compensation. Autosomal genes called autosomal signal elements (ASE) communicate the autosomal dose and counteract XSEs to activate
xol-1. In XO embryos,
xol-1 is active, male development ensues, and the dosage compensation machinery is turned off. How XSEs and ASEs oppose each other remains mysterious.
The XSE
sex-1 encodes a nuclear hormone receptor (NHR) that associates with the
xol-1 promoter in vivo to repress its transcription. To learn how
sex-1 functions and ultimately how XSEs oppose ASEs, SEX-1 binding sites on the
xol-1 promoter have been identified in vitro, revealing that SEX-1 association with
xol-1 is direct and mediated by prototypical NHR response elements. Further studies are validating SEX-1 binding sites in vivo and determining how these multiple sites contribute to dose dependent
xol-1 repression. Secondly, a new deletion allele of
sex-1,
y424, resulting in enhanced XX lethality, has allowed further dissection of
sex-1"s roles.
xol-1(-) only partially rescues
sex-1(
y424), and RNAi of a downstream dosage compensation gene is synergistically lethal with
xol-1(-)
sex-1(
y424), suggesting
sex-1 acts both upstream and downstream of
xol-1. In addition, mutations in ASEs further rescue XX viability of
xol-1(-)
sex-1(
y424) double mutants, suggesting a role for ASEs downstream of
xol-1 as well.
Although the only characterized SEX-1 function is transcriptional repression, SEX-1 possesses the conserved AF-2 domain necessary for transcriptional activation. To examine if activation by SEX-1 is involved in sex determination, transgenes encoding SEX-1 lacking the AF-2 domain were made. If truncated SEX-1 cannot fully rescue
sex-1(
y424), transcriptional activation may be required for control of
sex-1 target genes used in downstream events. These results and continuing genetic and biochemical studies will help define how
sex-1, and potentially ASEs, carry out multiple roles.