Domain C2 of tRNA-guanine transglycosylase is formed by a four-stranded anti-parallel beta-sheet lined with two alpha helices. It has conserved basic residues on the surface of the beta-sheets as does the C-terminal domain PUA, Pfam:PF01472. The catalytic domain, TGT has conserved basic residues on the outer surface of the N-terminal three-stranded beta sheet, which closes the barrel, and it is postulated that these basic residues from the three domains form a continuous, positively charged patch to which the tRNA binds [1].

1: Crystal structure of archaeosine tRNA-guanine transglycosylase. Ishitani R, Nureki O, Fukai S, Kijimoto T, Nameki N, Watanabe M, Kondo H, Sekine M, Okada N, Nishimura S, Yokoyama S; J Mol Biol 2002;318:665-677. PMID:12054814