The basic-helix-loop-helix-PAS (bHLH-PAS) proteins are a family of transcription factors that mediate diverse processes, including cellular adaptations to environmental signals and developmental cell fate decisions. The C.elegans genome encodes 5 bHLH-PAS proteins. These include AHR-1, HIF-1, and AHA-1. The AHA-1 protein dimerizes with multiple bHLH-PAS partners and is expressed in most, if not all, somatic cells. Although
ahr-1,
hif-1 double mutants are viable, animals homozygous for null mutations in
aha-1arrest development as young larvae. To understand the essential functions of
aha-1, we examined the expression of the two remaining bHLH-PAS genes, T01D3.2 and
cky-1/C15C8.2. A T01D3.2:GFP reporter is expressed in two neurons. CKY-1:GFP is expressed in most non-neuronal pharyngeal cells. These data suggested two non-exclusive models to explain the essential developmental function of AHA-1: i) AHA-1 might form a complex with CKY-1 in pharyngeal cells; or ii) AHA-1 might function as a homodimer or independently of other bHLH-PAS proteins. In support of the first model, AHA-1 and CKY-1 form a DNA-binding complex in vitro, and AHA-1 is localized to the nucleus of cells that express CKY-1:GFP. To determine whether expression of AHA-1 in the pharynx could rescue
aha-1-deficient animals, we constructed a
cky-1:
aha-1 chimeric gene, in which
cky-1 5' regulatory sequences directed the expression of the
aha-1 transcript. We injected
cky-1:
aha-1 into the
aha-1mutant background, thereby creating animals mosaic for AHA-1 expression. The transgenic animals were viable and fertile. We conclude that AHA-1 function is essential, and we propose that AHA-1 and CKY-1 have a role in pharyngeal function.