Structural aspects of nucleotide ligand binding by a bacterial 2H phosphoesterase

Myllykoski, Matti; Kursula, Petri

The original paper describing LigT identified it as having enzymatic activity resembling tRNA ligases [10] . These activities included the 3 0 -phosphodiesterase activity towards the 2 0 ,3 0cyclic phosphate present in the 3 0 -terminus of the 5 0 -half of the cleaved tRNA molecule, and the subsequent ligation of the 3 0 and 5 0 halves with a 2 0 -5 0 -phosphodiester bond between the 2 0 -phosphate group formed in the previous reaction and the 5 0 -hydroxyl group of the 3 0 -half of the cleaved tRNA molecule [10] . The ligation was later found to be reversible, as the enzyme additionally functions as a 2 0 -5 0 -phosphodiesterase [3, 9] . Recently, however, concomitant with the publication of the first E. coli LigT structure, this view was challenged, as LigT did not appear to ligate short 10-nucleotide RNA oligomers with 2 0 ,3 0 -cyclic phosphate and 5 0hydroxyl ends, but only acted on the cyclic phosphate [22] . Thus, LigT was claimed to be a CNPase rather that RNA ligase. It should be noted that the experimental conditions in the different studies varied, and it is hard to draw a definite conclusion at this point. Our structural data do highlight close structural similarities of bacterial LigT to RNA ligases and clear differences with respect to the vertebrate CNPase active site.