Molecular mechanism of substrate recognition and specificity of tRNAHis guanylyltransferase during nucleotide addition in the 3′–5′ direction

Abstract

The tRNAHis guanylyltransferase (Thg1) transfers a guanosine triphosphate (GTP) in the 3′–5′ direction onto the 5′-terminal of tRNAHis, opposite adenosine at position 73 (A73). The guanosine at the −1 position (G−1) serves as an identity element for histidyl-tRNA synthetase. To investigate the mechanism of recognition for the insertion of GTP opposite A73, first we constructed a two-stranded tRNAHis molecule composed of a primer and a template strand through division at the D-loop. Next, we evaluated the structural requirements of the incoming GTP from the incorporation efficiencies of GTP analogs into the two-piece tRNAHis. Nitrogen at position 7 and the 6-keto oxygen of the guanine base were important for G−1 addition; however, interestingly, the 2-amino group was found not to be essential from the highest incorporation efficiency of inosine triphosphate. Furthermore, substitution of the conserved A73 in tRNAHis revealed that the G−1 addition reaction was more efficient onto the template containing the opposite A73 than onto the template with cytidine (C73) or other bases forming canonical Watson–Crick base-pairing. Some interaction might occur between incoming GTP and A73, which plays a role in the prevention of continuous templated 3′–5′ polymerization. This study provides important insights into the mechanism of accurate tRNAHis maturation.