ABSTRACT
In modern byperthennophiles, stable RNAs might be protected against thennodegradation by their elaborated tertiary structures, interactions with proteins, and possibly methylation of the critical2' OH at the surface of the molecule. As a matter of fact, extensive ribose methylation of tRNA has been reported in hyperthennophiles (Edmonds et al., 1991). Similarly, the problem ofmRNA thennodegradation in present-day prokaryotes could be bypassed by the high turnover of these short-lived molecules, which is made possible by the coupling of transcription and translation (Forterre, 1995). This could even explain why eukaryotes, which require stable mRNA, cannot adapt to hyperthennophilic conditions (Forterre, 1995). However, advocates of the hot origin of life hypothesis have yet to propose reasonable mechanisms that might have prevented RNA thennodegradation in primitive RNA-based cells without paralysing ribozyme activities, the latter being mainly linked to the critical reactivity of the 2' ribose hydroxyl. Otherwise, it would remain more appropriately cautious to conclude like Joyce (1988) that 'without the benefit of evolutionary improvement, such an entity [a primitive organism which has not yet embarked on a particular evolutionary pathway] must have been biochemically inept in the extreme'.
