Characterization of an extremely thermostable restriction enzyme, PspGI, from a Pyrococcus strain and cloning of the PspGI restriction-modification system in Escherichia coli.

An extremely thermostable restriction endonuclease, PspGI, was purified from Pyrococcus sp. strain GI-H. PspGI is an isoschizomer of EcoRII and cleaves DNA before the first C in the sequence 5' CCWGG 3' (W is A or T). PspGI digestion can be carried out at 65 to 85 degrees C. To express PspGI at high levels, the PspGI restriction-modification genes (pspGIR and pspGIM) were cloned in Escherichia coli. M.PspGI contains the conserved sequence motifs of alpha-aminomethyltransferases; therefore, it must be an N4-cytosine methylase. M.PspGI shows 53% similarity to (44% identity with) its isoschizomer, M.MvaI from Micrococcus variabilis. In a segment of 87 amino acid residues, PspGI shows significant sequence similarity to EcoRII and to regions of SsoII and StyD4I which have a closely related recognition sequence (5' CCNGG 3'). PspGI was expressed in E. coli via a T7 expression system. Recombinant PspGI was purified to near homogeneity and had a half-life of 2 h at 95 degrees C. PspGI remained active following 30 cycles of thermocycling; thus, it can be used in DNA-based diagnostic applications.