Archaeosine Modification of Archaeal tRNA: Role in Structural Stabilization

Archaeosine (G(+)) is a structurally complex modified nucleoside found quasi-universally in the tRNA of Archaea and located at position 15 in the dihydrouridine loop, a site not modified in any tRNA outside the Archaea. G(+) is characterized by an unusual 7-deazaguanosine core structure with a formamidine group at the 7-position. The location of G(+) at position 15, coupled with its novel molecular structure, led to a hypothesis that G(+) stabilizes tRNA tertiary structure through several distinct mechanisms. To test whether G(+) contributes to tRNA stability and define the biological role of G(+), we investigated the consequences of introducing targeted mutations that disrupt the biosynthesis of G(+) into the genome of the hyperthermophilic archaeon Thermococcus kodakarensis and the mesophilic archaeon Methanosarcina mazei, resulting in modification of the tRNA with the G(+) precursor 7-cyano-7-deazaguansine (preQ(0)) (deletion of arcS) or no modification at position 15 (deletion of tgtA). Assays of tRNA stability from in vitro-prepared and enzymatically modified tRNA transcripts, as well as tRNA isolated from the T. kodakarensis mutant strains, demonstrate that G(+) at position 15 imparts stability to tRNAs that varies depending on the overall modification state of the tRNA and the concentration of magnesium chloride and that when absent results in profound deficiencies in the thermophily of T. kodakarensis. IMPORTANCE Archaeosine is ubiquitous in archaeal tRNA, where it is located at position 15. Based on its molecular structure, it was proposed to stabilize tRNA, and we show that loss of archaeosine in Thermococcus kodakarensis results in a strong temperature-sensitive phenotype, while there is no detectable phenotype when it is lost in Methanosarcina mazei. Measurements of tRNA stability show that archaeosine stabilizes the tRNA structure but that this effect is much greater when it is present in otherwise unmodified tRNA transcripts than in the context of fully modified tRNA, suggesting that it may be especially important during the early stages of tRNA processing and maturation in thermophiles. Our results demonstrate how small changes in the stability of structural RNAs can be manifested in significant biological-fitness changes.