CHARACTERIZATION OF A PLASMID CARRIED BY METHANOBACTERIUM-THERMOAUTOTROPHICUM ZH3, A METHANOGEN CLOSELY-RELATED TO METHANOBACTERIUM-THERMOAUTOTROPHICUM MARBURG

Strain ZH3, a hydrogenotrophic, thermophilic methanogenic archaeon was isolated from a sewage sludge digester. Based on its phenotypic prope rties and its 16S rRNA sequence (99.9% similarity), strain ZH3 is clos ely related to Methanobacterium thermoautotrophicum Marburg. The newly isolated organism harbored pME2200, a cryptic 6.2 kb plasmid, which i s related to, but not identical with the previously characterized cryp tic 4.4 kb plasmid pME2001 of strain Marburg. Plasmid pME2200 was char acterized by restriction mapping and by Southern blot and DNA sequence analysis and compared to plasmid pME2001. The two elements are compos ed of a presumably identical, 3.8 kb backbone structure. In addition t o the common backbone, both plasmids carry a region of high interplasm id homology, which in pME2200 was interspersed with three segments not present in pME2001. These interspersed segments did not encode potent ial open reading frames. One of them contained a singular restriction site which may be of use for the construction of potential cloning vec tors that are undamaged in essential plasmid functions.