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Anti-poly(ADP-ribose) glycohydrolase immunoblotting of a lysate from Sulfolobus solfataricus (strain MT-4) cells showed a main intense signal close to the 37 kDa protein marker. The immunoreactive protein was purified by electroelution and showed a hydrolysing activity towards oligomers (1- 6 residues) of ADP-ribose similar to eukaryotc poly(ADP-ribose) glycohydrolase. This protein was characterized as it regards enzymatic inhibition by adenosine diphosphate- (hydroxymethyl)pyrrolidine-3,4-diol, a known inhibitor of eukaryotic poly(ADP-ribose) glycohydrolase, and by analysis of reaction products. ADP-ribose polymer electrophoresis and thin layer chromatography clearly showed that the enzyme was able to monomerize Sulfolobus solfataricus MT-4 (ADP-ribose)1-6, an oligomer recognized also by eukaryotic poly (ADP-ribose) glycohydrolases. Edman degradation of the purified protein allowed to determine a short N-terminal sequence: Met-Ile-Ser-Val-Ala. This pentapeptide was used for a blast search towards Sulfolobus solfataricus genomes. It gave evidence of a 40 kDa-protein present only in two strains (P2 and 98/2) of Sulfolobus solfataricus. Oligonucleotide primers drawn on the cDNA of human poly(ADP-ribose) glycohydrolase gave a fragment of the corresponding Sulfolobus solfataricus MT-4 gene overlapping the sequences from the genomes of Sulfolobus solfataricus P2 and 98/2. Translation of the sequence confirmed the occurrence of a region with some amino acids matching the human poly(ADP-ribose) glycohydrolase “signature”.