A new method for manipulating transgenes: engineering heat tolerance in a complex, multicellular organism.

Heat-shock proteins (hsps) are thought to protect cells against stresses, especially due to elevated temperatures. But while genetic manipulation of hsp gene expression can protect microorganisms and cultured metazoan cells against lethal stress, this has so far not been demonstrated in multicellular organisms. Testing whether expression of an hsp transgene contributes to increased stress tolerance is complicated by a general problem of transgene analysis: if the transgene cannot be targeted to a precise site in the genome, newly observed phenotypes may be due to either the action of the transgene or mutations caused by the transgene insertion. To study the relationship between heat tolerance and hsp expression in Drosophila melanogaster, we have developed a novel method for transgene analysis, based upon the site-specific FLP recombinase. The method employs site-specific sister chromatid exchange to create an allelic series of transgene insertions that share the same integration site, but differ in transgene copy number. Phenotypic differences between members of this series can be confidently attributed to the transgenes. Using such an allelic series and a novel thermotolerance assay for Drosophila embryos, we investigated the role of the 70 kD heat-shock protein, Hsp 70, in thermotolerance. At early embryonic stages, Hsp70 accumulation was rate-limiting for thermotolerance, and elevated Hsp70 expression increased survival at extreme temperatures. Our results provide an improved method for analyzing transgenes and demonstrate that, in Drosophila, Hsp70 is a critical thermotolerance factor. They show, moreover, that manipulating the expression of a single hsp can be sufficient to improve the stress tolerance of a complex multicellular organism.