Investigation on the Causes of Codon and Amino Acid Usages Variation Between ThermophilicAquifex aeolicusand MesophilicBacillus subtilis

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Abstract
Base composition, codon usages and amino acid usages have been analyzed by taking 529 orthologous sequences of Aquifex aeolicus and Bacillus subtilis, having different optimal growth temperatures. These two bacteria do not have significant difference in overall GC composition, but GC(1+2) and GC3 levels were found to vary significantly. Significant increments in purine content and GC3 composition have been observed in the coding sequences of Aquifex aeolicus than its Bacillus subtilis counterparts. Correspondence analyses on codon and amino acid usages reveal that variation in base composition actually influences their codon and amino acid usages. Two selection pressures acting on the nucleotide level (GC3 and purine enrichment), causes variation in the amino acid usage differently in different protein secondary structures. Our results suggest that adaptation of amino acid usages in coil structure of Aquifex aeolicus proteins is under the control of both purine increment and GC3 composition, whereas the adaptation of the amino acids in the helical region of thermophilic bacteria is strongly influenced by the purine content. Evolutionary perspectives concerning the temperature adaptation of DNA and protein molecules of these two bacteria have been discussed on the basis of these results.