Protein-synthesizing machinery of thermophilic bacteria.

Thermophilic bacteria are among the truly unique microorganisms inhabiting our unicellular kingdom. Attempts to explain their ability to proliferate at temperatures over 70 C have resulted in widespread investigation during the past two decades. (The precise upper temperature limit for bacterial growth is a controversial issue. For a recent discussion of this problem, see reference 10.) The literature in this field has been previously reviewed by Gaughran (27), Allen (3), and Koffler (30). Many of the early studies were focused on the enzymes and structural proteins (flagella) isolated from thermophilic bacteria. With a few exceptions (which may not be valid because of the impure preparations used), the results demonstrated that thermophile proteins were more thermostable than comparable preparations obtained from mesophiles. These findings implied that thermophily was achieved by virtue of inherent properties of thermophile cellular constituents, and was not merely the result of a favorable equilibrium existing between rapidly resynthesized and heat-denatured molecules. The argument for the presence of stabilizing factors or the absence of labilizing factors was not seriously challenged until Campbell and his coworkers isolated and studied a crystalline, heatstable a-amylase from Bacillus stearothermophilus (13, 14, 34, 35). In addition, this important work provided the first clue to a molecular basis for thermostability. The amino acid composition data revealed an unusually high proline content (14), resulting in a large negative optical rotation (35). The suggestion has been made that the thermostability of the protein molecule may be a consequence of the fact that the enzyme exists in an