HYPERTHERMOPHILES AND THE PROBLEM OF DNA INSTABILITY

Rates of chemical decomposition of DNA at the optimal growth temperatu res of hyperthermophiles seem incongruent with the requirements of acc urate genome replication. The peculiar physiology, ecology and phyloge ny of hyperthermophiles combine to suggest that these prokaryotes have solved a molecular problem (spontaneous loss of native DNA structure) of a magnitude that well-studied microorganisms do not face. The fail ure of DNA base composition to correlate with optimal growth temperatu re among hyperthermophiles provides indirect evidence that other mecha nisms maintain their chromosomal DNA in the duplex form. Studies in vi tro indicate that DNA primary structure is more difficult to maintain at extremely high temperature than is secondary structure, yet hyperth ermophiles exhibit only modest levels of spontaneous mutation. Radiati on sensitivity studies also indicate that hyperthermophiles repair the ir DNA efficiently in vivo, and underlying mechanisms are beginning to be examined. Several enzymes of DNA metabolism from hyperthermophilic archaea exhibit unusual biochemical features that may ultimately prov e relevant to DNA repair. However, genomic sequencing results suggest that many DNA repair genes of hyperthermophilic archaea may not be rec ognized because they are not sufficiently related to those of well-stu died organisms.