  A previously established multiscale population genetics model states that
fitness can be inferred from the physical properties of proteins under the
physiological assumption that a loss of stability by any protein confers the
lethal phenotype to an organism. Here we develop this model further by positing
that replication rate (fitness) of a bacterial or viral strain directly depends
on the copy number of folded proteins which determine its replication rate.
Using this model, and both numerical and analytical approaches, we studied the
adaptation process of bacteria and viruses at varied environmental
temperatures. We found that a broad distribution of protein stabilities
observed in the model and in experiment is the key determinant of thermal
response for viruses and bacteria. Our results explain most of the earlier
experimental observations: striking asymmetry of thermal response curves, the
absence of evolutionary trade-off which was expected but not found in
experiments, correlation between denaturation temperature for several protein
families and the Optimal Growth Temperature (OGT) of their host organisms, and
proximity of bacterial or viral OGTs to their evolutionary temperatures. Our
theory quantitatively and with high accuracy described thermal response curves
for 35 bacterial species using, for each species, only two adjustable
parameters, the number of replication rate determining genes and energy barrier
for metabolic reactions.
