Temperature dependence of the activity of DNA-modifying enzymes: endonucleases and DNA ligase

The activities of 17 endonucleases: the restriction endonucleases AvaI, Bam HI, EcoRI, HindIII, PstI and SalI, which cleave pBR322 DNA once: AluI, AvaII, CfoI, HaeIII, HhaI, HinfI, HpaII and TaqI, which cut pBR322 DNA several times, and three 'unspecific' nucleases (S1 nuclease, staphylococcal nuclease and DNase I from bovine pancreas) were determined between 0 degrees and 65 degrees C. The reaction was followed by the disappearance of covalently closed circular pBR322 DNA, using the alkaline ethidium fluorescence assay of Morgan et al. [Nucleic Acids Res. (1979) 7, 547-594]; the activity of T4 DNA ligase was similarly measured by the conversion of nicked circular DNA to closed circular DNA. For each enzyme, small aliquots of the same solution were incubated at different temperatures simultaneously in a temperature gradient device, resulting in a high relative precision. The experimental results are summarized by the simplest possible theoretical description, using linear or exponential kinetics and apparent activation energies Ea for the enzymatic reaction, Ei for the enzyme inactivation and Ti for the inactivation temperature. To a good approximation these three parameters suffice for describing the temperature dependence of the activity of most of the enzymes.