Structure and Functions of Chromosomal and Extrachromosomal DNA in Halobacteria

The genomic DNA of Halobacterium halobium and several other extremely halophilic species is separated on bisacrylamide/malachitegreen columns into two fractions which differ considerably in their GC content. It is shown that the AT richer fraction exhibits a high degree of variability and often forms circular DNA elements of various sizes. The major circular DNA behaves like a plasmid in that it is present in a constant number of copies. Insertions of IS-like elements of different lengths seem to be responsible for the genetic variability. These insertions are reversible in some instances. This has allowed us to localize the genetic determinant for gas vesicle formation on the major circular DNA.