Pore-forming toxins (PFTs) represent the largest class of bacterial protein toxins and often play an important role in the bacterial pathogenesis. The pores formed by these proteins in the membrane of target cells lead to ion imbalances and signal transduction activation that represent a tremendous stress on cell. We are studying pore-forming Crystal (Cry) proteins made by the bacterium Bacillus thuringiensis (Bt) to understand how cellular factors and pathways respond to PFTs. In particular, our lab is interested in understanding what genes and pathways act in concert with the pore to undermine cellular health and integrity. The prediction is elimination of such gene and pathways will lead to animals resistant to the PFTs. A previous genetic screen in the laboratory for C. elegans resistance mutants to the PFT Cry5B led to the identification of glycolipid carbohydrates as key receptors for Cry5B toxin. These results, although important, did not actually point to broadly applicable mechanisms. Therefore, to try to get to these intoxication pathways, we screened for mutants resistant to a different PFT, Cry21A. These results will be presented here. Despite extensive screening, only one robust mutant resistant to Cry21A was found. Surprisingly this mutant presents an early stop codon in egl-9, which encodes for a key regulator of the hypoxia pathway. In an egl-9 mutant, the hypoxia-inducible factor (HIF-1) is stable and activates transcription of genes involved in cell survival under hypoxia. We found that egl-9 mutants are also resistant to various other Cry PFTs. These data suggest that repression of the hypoxia pathway by egl-9 makes animals more susceptible to PFTs. Loss-of-function mutation in egl-9 has already been reported to confer resistance to pathogenic attack (Darby et al., 1999), therefore we tested egl-9 to different stresses. We found that egl-9 mutants are also resistant to heat shock, oxidative stress, pathogenic bacteria and have an extended lifespan. However, the dependence of these stress responses on HIF-1 can be diametrically opposite, suggesting that both stabilization and loss of function of HIF-1 lead to stress resistance, depending on the stress. We believe we have also identified a factor downstream of HIF-1 required for a normal response to PFTs. Our data show that HIF-1 levels play an important role on cellular stress resistance and longevity and that maintaining a proper and dynamic balance of HIF-1 activity is crucial for proper survival of the animal under varying conditions. Lethal paralysis of Caenorhabditis elegans by Pseudomonas aeruginosa. 1999. Darby C, Cosma CL, Thomas JH, Manoil C. Proc Natl Acad Sci U S A. 96:15202-7.