Transcriptome and growth efficiency comparisons of recombinant thermophiles that produce thermolabile and thermostable proteins: implications for burden-based selection of thermostable proteins.

Geobacillus kaustophilus is a thermophilic bacterium that grows at temperatures ranging between 42 and 74 °C. Here, we modified this organism to produce the thermolabile protein (PyrFA) or its thermostable variant (PyrFV) and analyzed the transcriptome and growth efficiency profiles of the resultant strains. In the producer of PyrFA, the transcriptome profile was changed to facilitate ATP synthesis from NADH without pooling reduced quinones. This change implies that PyrFA production at elevated temperatures places an energy burden on cells potentially to maintain protein homeostasis. This was consistent with the observation that the PyrFA producer grew slower than the PyrFV producer at > 45 °C and had a lower cellular fitness. Similar growth profiles were also observed in the PyrFA and PyrFV producers derived from another thermophile (Geobacillus thermodenitrificans) but not in those from Escherichia coli at 30 °C. Thus, we suggest that the production of thermolabile proteins impairs host survival at higher temperatures; therefore, thermophiles are under evolutionary selection for thermostable proteins regardless of whether their functions are associated with survival advantages. This hypothesis provides new insights into evolutionary protein selection in thermophiles and suggests an engineering approach to select thermostable protein variants generated via random gene mutagenesis.