Biochemical and functional characterization of an exonuclease from Chaetomium thermophilum

Exonucleases are often found associated with polymerase or helicase domains in the same enzyme or can function as autonomous entities to maintain genome stability. Here, we uncovered Chaetomium thermophilum RecQ family proteins that also have exonuclease activity in addition to their main helicase function. The novel exonuclease activity is separate from the helical core domain and coexists with the latter two enzymatic activities on the same polypeptide. The CtRecQ^121-366 exonuclease region performs independently as an exonuclease. We describe its catalytic mechanism and biological characteristics. We demonstrate unequivocally that CtRecQ^121-366 exclusively displays exonuclease activity and that this activity has a 3'-5' polarity that can both hydrolyze ssDNA and cleave dsDNA substrates. The hydrolytic activity of majority exonuclease is driven by bimetal ions, and this appears to be the case for the CtRecQ^121-366 exonuclease as well. Additionally, the maximum activity of CtRecQ^121-366 was observed at pH 8.0-9.0, low salt with Mg²⁺. The two helices in the structure, a6 and a7, play significant roles in the execution by anticipating their shape and changing essential amino acids.