Correlation between the stability of tRNA tertiary structure and the catalytic efficiency of a tRNA‐modifying enzyme, archaeal tRNA‐guanine transglycosylase

In many archaeal tRNAs, archaeosine is found at position 15. During archaeosine biosynthesis, archaeal tRNA-guanine transglycosylase (ArcTGT) first replaces the guanine base at position 15 with 7-cyano-7-deazaguanine (preQ(0)). In this study, we investigated whether modified nucleosides in tRNA substrates would affect ArcTGT incorporation of preQ(0). We prepared a series of hypomodified tRNAs((GGA))(Ser) from Escherichiacoli strains lacking each tRNA-modifying enzyme. Measurement of ArcTGT kinetic parameters with the various tRNAs((GGA))(Ser) as substrates showed that the K-m decreased due to the lack of modified nucleosides. The tRNAs((GGA))(Ser) melting profiles resulted in experimental evidence showing that each modified nucleoside in tRNA((GGA))(Ser) enhanced tRNA stability. Furthermore, the ArcTGT K-m strongly correlated with the melting temperature (T-m), suggesting that the unstable tRNA containing fewer modified nucleosides served as a better ArcTGT substrate. These results show that preQ(0) incorporation into tRNA by ArcTGT takes place early in the archaeal tRNA modification process.