Master keys to DNA replication, repair, and recombination from the structural biology of enzymes from thermophiles

Microorganisms that are able to grow at temperatures above 90C are defi ned as hyperthermophiles, the majority of which are classifi ed as archaea [1]. Genes in these thermophiles encode proteins with high thermal stability even in the form of recombinant proteins expressed in bacteria, and thus provide advantages for characterizing protein interactions, conformations, and structures. In fact, most microbial responses to the environment involve reversible protein complexes and dynamic conformations that can be extremely challenging to study in mesophilic organisms but can often be kinetically trapped for hyperthermophiles. Such hyperthermophiles therefore not only aid crystallizations for x-ray structure determinations, but furthermore may provide a 1000-fold kinetic advantage for kinetically trapping the dynamic conformations and interactions responsible for most of the protein complexes and machines controlling microbial cell biology. Recently both x-ray structures and x-ray scattering in solution measurements or small angle x-ray scattering (SAXS) [2] of thermophilic proteins have dramatically improved our understanding of life and cell biology at the molecular level. This review focuses on our recent studies plus closely related literature results on thermophilic proteins that act in DNA replication, repair, and recombination as these are prototypical systems for understanding biologically important, dynamic protein interactions and conformations necessary for cells to survive, grow, and divide. We believe that an understanding of the structure and function of these thermophilic proteins is providing us with master keys [3] to the molecular mechanisms underlying reversible protein interactions and functional conformations critical for genome maintenance and cellular responses to endogenous and environmental stress.