Nuclease TT1 from Thermus thermophilus HB8 has an endonuclease activity preferential to circular DNAs

Homogeneously purified nuclease TT1 from Thermus thermophilus HB8 is known as an exonuclease to produce 5'-mononucleotides. Besides the exonuclease activity, nuclease TT1 also possesses endonuclease activity preferential to superhelical (form I) and single-stranded circular DNA. Although the rate of cleavage is slower than that of form I, covalently closed circular DNA (form I') is also cleaved. Form I DNA was nicked to yield relaxed circles (form II) first, and was then nicked at the opposite site to yield unit length linear DNA (form III) which was subsequently hydrolyzed to 5'-mononucleotides exonucleolytically. Both endo- and exo-nuclease activities co-migrate on polyacrylamide gels. The general properties of the endonuclease activity are very similar to those of the exonuclease activity. The temperature optimum for endonuclease activity was 85 degrees C. The pH-optimum was in pH-range from 7.5-9.1. The enzyme was active over a wide range of Mg2+ concentrations (2.5-125 mM), and was inhibited by EDTA. A linear substrate such as (dT)8 was a competitive inhibitor for this endonuclease activity.