Abstract
Genomic correlates of evolutionary adaptation to very low or very high optimal growth temperature (OGT) values have been the subject of many studies. Whereas these provided a protein-structural rationale of the activity and stability of globular proteins/enzymes, the point has been neglected that adaptation to extreme temperatures could also have resulted from an increased use of intrinsically disordered proteins (IDPs), which are resistant to these conditions in vitro. Contrary to these expectations, we found a conspicuously low level of structural disorder in bacteria of very high (and very low) OGT values. This paucity of disorder does not reflect phylogenetic relatedness, i.e. it is a result of genuine adaptation to extreme conditions. Because intrinsic disorder correlates with important regulatory functions, we asked how these bacteria could exist without IDPs by studying transcription factors, known to harbor a lot of function-related intrinsic disorder. Hyperthermophiles have much less transcription factors, which have reduced disorder compared to their mesophilic counterparts. On the other hand, we found by systematic categorization of proteins with long disordered regions that there are certain functions, such as translation and ribosome biogenesis that depend on structural disorder even in hyperthermophiles. In all, our observations suggest that adaptation to extreme conditions is achieved by a significant functional simplification, apparent at both the level of the genome and individual genes/proteins.
Highlights
Life has adapted to extreme conditions from sub-zero temperatures in sea ice of polar regions to boiling temperatures in hydrothermal vents [1,2]
Neither amino-acid composition, nor distribution of proteins in the CH-plot (Supplementary Figure S1) show a characteristic bias between the four groups, which suggests that disorder predictions by IUPred truly reflect the structural status of proteins encoded by genomes of bacteria of various optimal growth temperature (OGT) values
Because intrinsically disordered proteins (IDPs) often do not aggregate under high- or low-temperature conditions [28,33], and they can be effective in preventing other proteins from aggregation [31,32,34,35], it was expected that prokaryotes adapted to extremely low or extremely high temperatures have relied on IDPs in their adaptation to these extreme temperatures
Summary
Life has adapted to extreme conditions from sub-zero temperatures in sea ice of polar regions to boiling temperatures in hydrothermal vents [1,2]. The underlying molecular mechanisms have been studied either by comparing the structures of proteins isolated from organisms that thrive at low (psychrophilic), moderate (mesophilic) or high (thermophilic) temperatures [5,6,7,8], or analyzing sequences of the respective genomes/ proteomes [9,10,11,12] It appears that proteins of vastly different optimal temperatures show only subtle differences in structure, and their adaptation relies on an interplay of various factors affecting stability, such as hydrophobicity, H-bonds, structural cavities, ion-pairs, and secondary structural elements, including surface loops [13]. Compositional differences contribute to thermal adaptation through fine-tuning stability, flexibility and specific activity of proteins [6], by making them in general more rigid and more stable to thermal unfolding with increasing growth temperatures
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