Rat liver DNA ligases: catalytic properties of a novel form of DNA ligase

A novel form of rat liver DNA ligase (molecular mass 100 kDa) can be differentiated from DNA ligase I by several biochemical parameters. It is a more heat-labile enzyme and unable to join blunt-ended DNA, even in the presence of poly(ethylene glycol) concentrations which stimulate such joining by DNA ligase I and T4 DNA ligase. It also lacks the AMP-dependent nicking/closing reaction, which is a property of all other DNA ligases tested so far, including DNA ligase I from rat liver. Both rat liver DNA ligases were inhibited by deoxyadenosinetriphosphate, however this inhibition was competitive with respect to ATP, for DNA ligase I (K(i) 22-mu-M) and non-competitive for the 100-kDa DNA ligase (K(i) 170-mu-M). These results support the idea that, when compared with other DNA ligases, the novel form of DNA ligase has a unique AMP-binding site, may have an absolute requirement for single-strand breaks and, furthermore, may have an altered reaction mechanism to that which is conserved from bacteriophage to mammalian DNA ligase I.