Thermophilic bacteria strictly obey Szybalski's transcription direction rule and politely purine-load RNAs with both adenine and guanine

When transcription is to the right of the promoter, the "top," mRNA-synonym ous strand of DNA tends to be purine-rich. When transcription is to the lef t of the promoter, the top, mRNA-template strand tends to be pyrimidine-ric h. This transcription-direction rule suggests that there has been an evolut ionary selection pressure for the purine-loading of RNAs. The politeness hy pothesis states that purine-loading prevents distracting RNA-RNA interactio ns and excessive formation of double-stranded RNA, which might trigger vari ous intracellular alarms. Because RNA-RNA interactions have a distinct entr opy-driven component, the pressure For the evolution of purine-loading migh t be greater in organisms living at high temperatures. In support of this, we find that Chargaff differences (a measure of purine-loading) are greater in thermophiles than in nonthermophiles and extend to both purine bases. I n thermophiles the pressure to purine-load affects codon choice, indicating that some Features of their amino acid composition (e.g., high levels of g lutamic acid) might reflect purine-loading pressure (i.e., constraints on m RNA) rather than direct constraints on protein structure and function.