On the origin of genomic adaptation at high temperature for prokaryotic organisms

Abstract

For a long time, the central issue of evolutionary genomics was to find out the adaptive strategy of nucleic acid molecules of various microorganisms having different optimal growth temperatures ( T opt). Long-standing controversies exist regarding the correlations between genomic G + C content and T opt, and this debate has not been yet settled. We address this problem by considering the fact that adaptation to growth at high temperature requires a coordinated set of evolutionary changes affecting: (i) nucleic acid thermostability and (ii) stability of codon–anticodon interactions. In the present study, we analyzed 16 prokaryotic genomes having intermediate G + C content and widely varying optimal growth temperatures. Results show that elevated growth temperature imposes selective constraints not only on nucleic acid level but also affects the stability of codon–anticodon interaction. We observed a decrease in the frequency of SSC and SSG codons with the increase in T opt to avoid the formation of side-by-side GC base pairs in the codon–anticodon interaction, thereby making it impossible for a genome to increase GC composition uniformly through the whole coding sequence. Thus, we suggest that any attempt to obtain a generalized relation between genomic GC composition and optimal growth temperature would hardly evolve any satisfactory result.