Doing it in reverse: 3′-to-5′ polymerization by the Thg1 superfamily

Abstract

The tRNA His guanylyltransferase (Thg1) family of enzymes comprises members from all three domains of life (Eucarya, Bacteria, Archaea). Although the initial activity associated with Thg1 enzymes was a single 39-to-59 nucleotide addition reaction that specifies tRNAHis identity in eukaryotes, the discovery of a generalized base pair-dependent 39-to-59 polymerase reaction greatly expanded the scope of Thg1 family-catalyzed reactions to include tRNA repair and editing activities in bacteria, archaea, and organelles. While the identification of the 39-to-59 polymerase activity associated with Thg1 enzymes is relatively recent, the roots of this discovery and its likely physiological relevance were described~30 yr ago. Here we review recent advances toward understanding diverse Thg1 family enzyme functions and mechanisms. We also discuss possible evolutionary origins of Thg1 family-catalyzed 39-to-59 addition activities and their implications for the currently observed phylogenetic distribution of Thg1-related enzymes in biology.

Structural basis of reverse nucleotide polymerization

“…Stacking interaction between His154 (Fig. S7), as part of the eukaryotic-specific sequence motif H 154 INNLY (30), and G36 facilitates specific recognition of the purine base, and a corresponding His154Ala mutation decreases guanylylation activity to 50% (Fig. 3E).…”

Life without post-transcriptional addition of G−1: two alternatives for tRNAHis identity in Eukarya

“…1; Jackman et al 2012). Previous studies focused on A. castellanii, which lacks a bona fide Thg1 capable of adding G −1 to tRNA His , but encodes two related family members, which are Thg1-like proteins (TLPs) that apparently function in other processes beyond tRNA His metabolism (Abad et al 2011;Rao et al 2011).…”

“…Although numerous examples of eukaryotes that lack Thg1 are found among lower eukaryotic genomes (Jackman et al 2012), among well-sequenced metazoa there is only a single organism identified so far that lacks an identifiable Thg1 enzyme in its genome, which is Caenorhabditis elegans (Fig. 1).…”

In vitro substrate specificities of 3′–5′ polymerases correlate with biological outcomes of tRNA 5′‐editing reactions

“…Based on sequence similarity, 3'-5' polymerases are further divided into two distinct sub-families: the tRNA His guanylyltransferases (Thg1) and the Thg1-Like Proteins (TLPs) (3). Thg1 enzymes are strictly eukaryotic and their biological function, addition of an essential 5'-guanosine (G −1 ) to tRNA His , is well-established (4-6).…”