The lack of a stress response in Hydra oligactis is due to reduced hsp70 mRNA stability.

Synthesis and degradation of hsp70 mRNA was examined and compared in Hydra species living in different habitats and showing different heat-shock response. Hydra oligactis is restricted to habitats of low temperature and relatively stable pH. We have shown previously that this species is unable to acquire thermotolerance [Bosch, T., Krylow, S., Bode, H. & Steele, R. (1988) Proc. Natl. Acad. Sci. USA 85, 7927-7931] and synthesizes significantly less heat-shock protein and hsp70 mRNA [Gellner, K., Praetzel, G. & Bosch, T. C. G. (1992) Eur J. Biochem. 210, 683-691] in response to stress than related species, such as Hydra bulgaris or Hydra magnipapillata, which are adapted to habitats of wide temperature range and variable water quality. To examine the mechanisms responsible for the differential heat-shock responses in these species, a construct containing H. magnipapillata hsp70 regulatory sequences fused to firefly luciferase was introduced into H. oligactis and H. magnipapillata polyps, and expression of luciferase examined. The results showed that luciferase can be expressed equally well in a heat-inducible manner in both species, suggesting that H. oligactis heat-shock factor can interact with H. magnipapillata heat-shock elements. Northern blots of alpha-amanitin-treated polyps demonstrated that the half-life of hsp70 mRNA in heat-shocked H. oligactis is drastically shorter than in H. magnipapillata. Thus, differences in hsp70 mRNA stability appear to be responsible for the habitat-correlated differences in the stress response in Hydra species.