Compositional changes in RNA, DNA and proteins for bacterial adaptation to higher and lower temperatures.

It is known that in thermophiles the G+C content of ribosomal RNA linearly correlates with growth temperature, while that of genomic DNA does not. Although the G+C contents (singlet) of the genomic DNAs of thermophiles and methophiles do not differ significantly, the dinucleotide (doublet) compositions of the two bacterial groups clearly do. The average amino acid compositions of proteins of the two groups are also distinct. Based on these facts, we here analyzed the DNA and protein compositions of various bacteria in terms of the optimal growth temperature (OGT). Regression analyses of the sequence data for thermophilic, mesophilic and psychrophilic bacteria revealed good linear relationships between OGT and the dinucleotide compositions of DNA, and between OGT and the amino acid compositions of proteins. Together with the above-mentioned linear relationship between ribosomal RNA and OGT, the DNA and protein compositions can be regarded as thermostability measures for RNA, DNA and proteins, covering a wide range of temperatures. Both the DNA and proteins of psychrophiles apparently exhibit characteristics diametrically opposite to those of thermophiles. The physicochemical parameters of dinucleotides suggested that supercoiling of DNA is relevant to its thermostability. Protein stability in thermophiles is realized primarily through global changes that increase charged residues (i.e., Glu, Arg, and Lys) on the molecular surface of all proteins. This kind of global change is attainable through a change in the amino acid composition coupled with alterations in the DNA base composition. The general strategies of thermophiles and psychrophiles for adaptation to higher and lower temperatures, respectively, that are suggested by the present study are discussed.