PubMed1976
1) The adaptive system of thermophilic bacteria, as demonstrated with B. caldotenax, seems to be suitable to produce thermophilic and mesophilic enzymes for comparative studies. 2) If it may be assumed that the extensive homologies in the N-terminal sequences of the LDHs also extend over the entire polypeptide chain, comparison of these sequences together with investigation on the 3-dimensional structure offer the possibility of elucidating those structural details which may be responsible for thermostability and the other thermophilic properties. However, the difficulty still remains that the latter may be obscured by differences not related to thermostability etc. Neverthless it may be hoped that comparison of the full sequences of not only the LDHs but also of a sufficient number of other enzymes of the same system will yield such details. 3) A further interesting goal with respect to the mechanism of enzyme adaptation would be reached if the differences in amino acid sequence of thermophilic and mesophilic LDH enzymes would throw light on the type of the amino acids always being exchanged. Here from the very hypothetical point of view the question arises as to whether the bacterial cell during the metabolic adaptation process or even by mutation/selection is able to modify just those few amino acid residues thermodynamically important for thermostability. Alternatively: does there exist a "rule" by which certain amino acid residues are invariably exchanged on a change for thermophilic to mesophilic enzymes? 4) Problems not mentioned here arise with B. stearothermophilus, which can be adapted poorly via the spores or on intermediate temperatures. Of great importance, but also a special problem in these studies on thermophilic and mesophilic enzymes produced by the same bacterium are a) the characterization of the thermophilic (70 degrees or 55 degrees) and mesophilic (37 degrees) bacterial variants (in respect to type), b) the control of homogeneity of the bacterial culture (contamination, mixed population), c) proof of the genetic identity of the 70 degrees- (55 degrees-) and 37 degrees -variant of B. caldotenax and B. stearothermophilus, which differ greatly in their phenotypes, for example in their metabolism, cell- or colony merphology. The taxonomical-biochemical identity or also the identity of morphology of the sporangia, since this should be an expression of the temperature dependent phenotype, cannot be used unconditionally as criteria of identity. Criteria such as the presence of identical enzymes in both variants or the identity of the genome (use of genetic markers, anlaysis of the DNA) are more reliable. Experiments with both variants of B. caldotenax demonstrated an identically high content of cytosine plus guanine in their DNA: 62.2% in the thermophilic DNA and 66.8% in the mesophilic DNA. In the thermophilic B. stearothermophilus the C+G content of the DNA was 56.5% and in the mesophilic variant 57.1%...
Adaptation, Physiological↗