O2 homeostasis is essential for the survival of all organisms. From bacteria to mammals, cellular O2 levels are carefully regulated. Low O2 (hypoxia) will endanger ATP generation by shutting down the oxidative phosphorylation. In mammals hypoxia-inducible factor (HIF-1) plays a central role in regulation of responses to low O2 levels. It consists of two bHLH-PAS transcriptional subunits, HIF-1a and HIF-1b. HIF-1b is better known as aryl hydrocarbon receptor nuclear translocator (ARNT). Under hypoxic conditions, HIF-1a protein is stabilized and dimerizes with ARNT to form an active transcription heterodimer (HIF-1), which in turn activates the transcription of downstream targets. The target gene products either promote O2 delivery or switch metabolism to low O2 consumption. Multiple mechanisms of O2 sensing have been proposed, but none have been vigorously tested in vivo (1). Here, we report that bHLH-PAS genes may mediate response to hypoxia in C. elegans. The C. elegans genome encodes 5 putative transcription factors that contain basic-helix-loop-helix and PAS domain motifs. Two of these genes, ahr-1
, encode the orthologs of the aryl hydrocarbon receptor and ARNT, respectively (2). Genetic analysis of these two genes has demonstrated that aha-1
is essential for viability, while ahr-1
is not (3,4). This indicates that AHA-1 may dimerize with other bHLH-PAS proteins to regulate developmentally important processes. Four lines of evidence suggest that AHA-1 and F38A6.3a may form hypoxia-responsive complex in C. elegans. First, AHA-1 and the F38A6.3a gene product can be co-immunoprecipitated when expressed in vitro. Second, F38A6.3a mRNA levels increase in hypoxic conditions. Third, we have identified potential target genes that are induced by hypoxia in N2, but not in animals homozygous for a deletion in F38A6.3a. Fourth, AHA-1 is expressed in many tissues (3, 4), and F38A6.3a:GFP is ubiquitously expressed. Further characterization of the mutant phenotype will be presented at the meeting. References: 1. Semenza GL. Perspectives on Oxygen Sensing. Cell (1999) 98(3): 281-4. 2. Powell-Coffman JA, Bradfield CA, Wood WB. Caenorhabditis elegans orthologs of the aryl hydrocarbon receptor and its heterodimerization partner with the aryl hydrocarbon receptor nuclear translocator. PNAS (1998), 95: 2844-2849. 3. Jiang H, Guo R, Powell-Coffman JA. An essential role for aha-1
, a PAS-domain-containing regulatory gene in C. elegans. 1999 International Worm Meeting, 438. 4. See other Powell-Coffman lab abstracts for this meeting.