Introduction Most of the organisms that are mesophiles thrive at optimum growth temperature (OGT) of 24-40°C. Organisms living at higher OGT of 50-70°C are thermophiles and at OGT of greater than 80°C are hyperthermophiles which include members of domains Archaea and Eubacteria. Archaea are probably the earliest living organisms that occupy diverse habitats and are accordingly identified as thermophiles, halophiles or psychrophiles etc. (Woese et al., 1990; Doolittle, 1995; Zlatanova, 1997; Makarova and Koonin, 2003). Since most of the Archaea and some Eubacteria (Kreil and Ouzounis, 2001; Bao et al., 2002) live under extreme conditions, certain characteristic features might have enabled them to survive in these environments. Some of these features are modifications in the metabolic pathways of synthesis of cofactors like heme, acetyl CoA, acyl CoA, and folic acid which are either greatly reduced or are eliminated in thermophiles because of constrains of high temperature. This trend is seen in Archaea 61 Journal of Cell and Molecular Biology 4: 61-69, 2005. Haliç University, Printed in Turkey. Nucleic acid stability in thermophilic prokaryotes: a review Seema Trivedi 1 *, Satyawada Rama Rao 2 and Hukam Singh Gehlot 2 1 Department of Zoology, J N Vyas University, Jodhpur (Raj.), India 2 Department of Botany, J N Vyas University, Jodhpur (Raj.), India (*author for correspondence) Received 09 February 2005; Accepted 11 May 2005 Abstract In order to survive at temperatures of ≥60°C, thermophilic prokaryotes (Archaea and Eubacteria) have adopted different strategies. These strategies include high CG content in the coding sequences, nucleotide arrangement of purine-purine and pyrimidine-pyrimidine, methylation of nucleotides, histone/histone like proteins, reverse gyrase, cations, etc., also provide thermal stability to genome. Strategies adopted at the level of DNA are naturally, though not universally, reflected in RNA in thermophiles. Increased purine load (particularly of adenine), preferential codon usage, post-transcriptional modifications etc. provide thermal stability. All these factors may differ from taxa to taxa as no single or all the factors together can be universally attributed for providing thermal stability to nucleic acids. Key Words: Archaea, thermophiles, stability, CG content, reverse gyrase Termofilik prokaryotlarda nükleik asit kararl›l›¤› Özet 60 0 C ve üzerindeki s›cakl›klarda, termofilik prokaryotlar (Archaea ve Eubacteria) farkl› stratejiler uygularlar. Kodlayan dizinlerde CG içeri¤i, purin – purin ve pirimidin – pirimidinlerin nükleotiotid düzenlenmesi, nükleotidlerin metilasyonu, histon / histon benzeri proteinler, ters giraz, katyonlar v.b. stratejiler genoma termal kararl›l›k sa¤larlar. DNA seviyesinde uygulanan stratejiler, do¤al olarak, üniversal olmamas›na ra¤men, termofillerde RNA ya yans›r. Artan purin yüklemesi (özellikle adenin), tercihli kodon kullan›m›, post – transkripsiyonel modifikasyonlar termal kararl›l›¤› sa¤larlar. Bütün bu faktörler, tek olarak veya bir arada nükleik asitlere termal kararl›l›k sa¤layabilece¤inden durumdan duruma farkl›l›k gösterebilirler. Anahtar sözcükler: Archae, termofiller, kararl›l›k, CG içeri¤i, ters giraz