Analysis of GTP addition in the reverse (3′–5′) direction by human tRNA(His) guanylyltransferase

Human tRNA(His) guanylyltransferase (HsThg1) catalyzes the 3′–5′ addition of guanosine triphosphate (GTP) to the 5′-end (−1 position) of tRNA(His), producing mature tRNA(His). In human cells, cytoplasmic and mitochondrial tRNA(His) have adenine (A) or cytidine (C), respectively, opposite to G(−1). Little attention has been paid to the structural requirements of incoming GTP in 3′–5′ nucleotidyl addition by HsThg1. In this study, we evaluated the incorporation efficiencies of various GTP analogs by HsThg1 and compared the reaction mechanism with that of Candida albicans Thg1 (CaThg1). HsThg1 incorporated GTP opposite A or C in the template most efficiently. In contrast to CaThg1, HsThg1 could incorporate UTP opposite A, and guanosine diphosphate (GDP) opposite C. These results suggest that HsThg1 could transfer not only GTP, but also other NTPs, by forming Watson–Crick (WC) hydrogen bonds between the incoming NTP and the template base. On the basis of the molecular mechanism, HsThg1 succeeded in labeling the 5′-end of tRNA(His) with biotinylated GTP. Structural analysis of HsThg1 was also performed in the presence of the mitochondrial tRNA(His). Structural comparison of HsThg1 with other Thg1 family enzymes suggested that the structural diversity of the carboxy-terminal domain of the Thg1 enzymes might be involved in the formation of WC base-pairing between the incoming GTP and template base. These findings provide new insights into an unidentified biological function of HsThg1 and also into the applicability of HsThg1 to the 5′-terminal modification of RNAs.