Occurrence of megaplasmids in halobacteria

Sixty‐five halobacteria, including culture collection and freshly isolated strains from widely differing geographical areas, were examined for the presence of high molecular weight plasmids by agarose gel electrophoresis. Seventy‐five per cent of all the strains were shown to harbour at least one plasmid. In the majority of strains three or four megaplasmids were detected. Approximate molecular weights of the plasmids were in the range < 100 to 300 megadaltons (Mdal). In most culture collection strains, two or three plasmids were demonstrated, except in two in which no plasmid was detected, and in two Haloarcula strains which were found to contain five and eight plasmids; four and six of the latter were more than 100 Mdal. No relationship between the plasmid profile of each strain and its taxonomic assignation nor its isolation source was found. Evidence is presented for the first time on the occurrence of megaplasmids in halobacteria.