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-synonymous strand of DNA tends to be purine-rich. When transcription is to the left of the promoter, the top, mRNA-template strand tends to be pyrimidine-rich. This transcription-direction rule suggests that there has been an evolutionary selection pressure for the purine-loading of RNAs. The politeness hypothesis states that purine-loading prevents distracting RNA-RNA interactions and excessive formation of double-stranded RNA, which might trigger various intracellular alarms. Because RNA-RNA interactions have a distinct entropy-driven component, the pressure for the evolution of purine-loading might 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. In thermophiles the pressure to purine-load affects codon choice, indicating that some features of their amino acid composition (e.g., high levels of glutamic acid) might reflect purine-loading pressure (i.e., constraints on mRNA) rather than direct constraints on protein structure and function.
Lao
P J
PJ
Department of Biochemistry, Queen's University, Kingston, Ontario, K7L 3N6, Canada.
Forsdyke
D R
DR
D000225:Adenine / Q000378:metabolism; D001482:Base Composition / Q000235:genetics; D003597:Cytosine Nucleotides / Q000235:genetics; D005798:Genes, Bacterial / Q000235:genetics; D006147:Guanine / Q000378:metabolism; D006150:Guanine Nucleotides / Q000235:genetics; D006358:Hot Temperature; D017017:Methanococcus / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D011687:Purines / Q000378:metabolism*; D012329:RNA, Bacterial / Q000235:genetics / Q000378:metabolism*; D018741:Replication Origin / Q000235:genetics; D014158:Transcription, Genetic / Q000235:genetics*
D003597:Cytosine Nucleotides; D006150:Guanine Nucleotides; D011687:Purines; D012329:RNA, Bacterial; D006147:Guanine; D000225:Adenine; C030985:purine