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 3′-to-5′ nucleotide addition reaction that specifies tRNA^His identity in eukaryotes, the discovery of a generalized base pair–dependent 3′-to-5′ 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 3′-to-5′ 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 3′-to-5′ addition activities and their implications for the currently observed phylogenetic distribution of Thg1-related enzymes in biology.