A 645-kDa proteasome was purified from Methanosarcina thermophila which had chymotrypsin-like and peptidylglutamyl-peptide hydrolase activities and contained α (24-kDa) and β (22-kDa) subunits. Processing of both subunits was suggested by comparison of N-terminal sequences with the sequences deduced from the α-and β-encoding genes (psmA and psmB). Alignment of deduced sequences for the α and β subunits revealed high similarity; however, the N-terminal sequence of the α subunit contained an additional 24 amino acids that were not present in the β subunit. The α and β subunits had high sequence identity with α- and β-type subunits of proteasomes from eucaryotic organisms and the distantly related archaeon Thermoplasma acidophilum. The psmB gene was transcribed in vive as a monocistronic message from a consensus archaeal promoter. The results suggest that proteasomes are more widespread in the Archaea than previously proposed. Southern blotting experiments suggested the presence of ubiquitin-like sequences in M. thermophila.
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