Template-dependent 3 ′ –5 ′ nucleotide addition is a shared feature of tRNA His guanylyltransferase enzymes from multiple domains of life

The presence of an additional 5′ guanosine residue (G-1 ) is a unique feature of tRNAHis . G-1 is incorporated posttranscriptionally in eukarya via an unusual 3′ –5′ nucleotide addition reaction catalyzed by the tRNAHis guanylyltransferase (Thg1). Yeast Thg1 catalyzes an unexpected second activity: Watson–Crick-dependent 3′ –5′ nucleotide addition that occurs in the opposite direction to nucleotide addition by all known DNA and RNA polymerases. This discovery led to the hypothesis that there are alternative roles for Thg1 family members that take advantage of this unusual enzymatic activity. Here we show that archaeal homologs of Thg1 catalyze G-1 addition, in vitro and in vivo in yeast, but only in a templated reaction, i.e. with tRNAHis substrates that contain a C73 discriminator nucleotide. Because tRNAHis from archaea contains C73 , these findings are consistent with a physiological function for templated nucleotide addition in archaeal tRNAHis maturation. Moreover, unlike yeast Thg1, archaeal Thg1 enzymes also exhibit a preference for template-dependent U-1 addition to A73 -containing tRNAHis . Taken together, these results demonstrate that Watson–Crick template-dependent 3′ –5′ nucleotide addition is a shared catalytic activity exhibited by Thg1 family members from multiple domains of life, and therefore, that this unusual reaction may constitute an ancestral activity present in the earliest members of the Thg1 enzyme family.