A PROTEASOME FROM THE METHANOGENIC ARCHAEON METHANOSARCINA-THERMOPHILA

A 645-kDa proteasome was purified from Methanosarcina thermophila whic h had chymotrypsin like and peptidylglutamyl-peptide hydrolase activit ies and contained alpha (24-kDa) and beta (22-kDa) subunits. Processin g of both subunits was suggested by comparison of N-terminal sequences with the sequences deduced from the alpha- and beta-encoding genes (p smA and psmB). Alignment of deduced sequences for the alpha and beta s ubunits revealed high similarity; however, the N-terminal sequence of the alpha subunit contained an additional 24 amino acids that were not present in the beta subunit. The alpha and beta subunits had high seq uence identity with alpha- and beta-type subunits of proteasomes from eucaryotic organisms and the distantly related archaeon Thermoplasma a cidophilum. The psmB gene was transcribed in vivo as a monocistronic m essage from a consensus archaeal promoter. The results suggest that pr oteasomes are more widespread in the Archaea than previously proposed. Southern blotting experiments suggested the presence of ubiquitin-lik e sequences in M. thermophila.