Temperature adaptive mutations

The number of thermally adaptive mutations resulting from directed evolution studies is too small at present to support a detailed statistical analysis. Here we summarize some properties of the mutations discovered in the studies reviewed above, and compare them to the amino-acid differences seen among naturally occurring enzymes that have adapted to different temperatures. Lists of the amino-acid substitutions discovered... 

A shift in temperature from 38 to 22 °C leads to desaturation of fatty acids in Anabaena variabilis [110], resulting in control of the fluidity of the plasma membrane. Mutants have been isolated in Synechocystis PCC 6803 that were defective in desaturation of fatty acids, and the growth rate of one of these mutants was much lower than that of the wild-type at 22 °C [112]. It turned out that the mutant strain had a mutation in the gene desA, and when the wild-type allele was introduced into the chilling-sensitive cyanobacterium Anacystis nidulans, it resulted in increasing the tolerance of that strain to low temperature [113]. These experiments nicely demonstrate the existence of a mechanism of adaptation to low temperature in a chilling-tolerant cyanobacterium. 

So what did happen to the thermostability of SSII during adaptation for activity in the cold P3C9 is less thermostable than its mesophilic counterpart SSII, with a half-life of inactivation at 70°C that decreased approximately threefold. However, there is no strict inverse correlation between stability and low temperature activity during the evolution. While low temperature activity was improved, thermal stability both increased and decreased, although the overall trend was toward decrease (Fig. 19). This suggests that, rather than being inversely coupled, thermal stability is essentially decoupled from low-temperature activity. And, since most mutations are deleterious, stability decreases when not subject to any selective pressure. 

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