a purified thermostable enzyme is derived from the thermophilic archaebacterium archaeoglobus fulgidus . the enzyme can be native or recombinant , is stable under pcr conditions and exhibits double strand specific exonuclease activity . it is a 3 ′– 5 ′ exonuclease and cleaves to produce 5 ′- mononucleotides . thermostable exonucleases are useful in many recombinant dna techniques , in combination with a thermostable dna polymerase like tag especially for nucleic acid amplification by the polymerase chain reaction .