Characterization of an ATP-dependent DNA ligase from the thermophilic archaeon Methanobacterium thermoautotrophicum

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Abstract
We report the production, purification and characterization of a DNA ligase encoded by the thermophilic archaeon Methanobacterium thermoautotrophicum. The 561 amino acid Mth ligase catalyzed strand-joining on a singly nicked DNA in the presence of a divalent cation (magnesium, manganese or cobalt) and ATP (K-m 1.1. mu M). dATP can substitute for ATP, but CTP, GTP, UTP and NAD(+) cannot. Mth ligase activity is thermophilic in vitro, with optimal nick-joining at 60 degrees C, Mutational analysis of the conserved active site motif I (KxDG) illuminated essential roles for Lys251 and Asp253 at different steps of the ligation reaction. Mutant K251A is unable to form the covalent ligase-adenylate intermediate (step 1) and hence cannot seal a 3'-OH/5'-PO4 nick. Yet, K251A catalyzes phosphodiester bond formation at a pre-adenylated nick (step 3), Mutant D253A is active in ligase-adenylate formation, but defective in activating the nick via formation of the DNA-adenylate intermediate (step 2), D253A is also impaired in phosphodiester bond formation at a pre-adenylated nick. A profound step 3 arrest, with accumulation of high levels of DNA-adenylate, could be elicited for the wild-type Mth ligase by inclusion of calcium as the divalent cation cofactor. Mth ligase sediments as a monomer in a glycerol gradient. Structure probing by limited proteolysis suggested that Mth ligase is a tightly folded protein punctuated by a surface-accessible loop between nucleotidyl transferase motifs III and IIIa.