Halobacterium strain 5 contains a plasmid which is correlated with the presence of gas vacuoles

Abstract

GAS vacuoles are subcellular inclusions in certain prokaryotic organisms which enclose a gas-filled space and consist of a proteinaceous membrane containing one or two very hydrophobic protein subunits¹. Gas vacuoles are, with few exceptions, limited to aquatic microorganisms², where it has been proposed either that they may be involved in regulating buoyancy, or that the light-scattering properties of the inclusions may protect the cells from the deleterious effects of high light intensities¹. Several species of Halobacterium possess gas vacuoles³, and in certain strains, forms which lack gas vacuoles arise at a relatively high frequency. The rate of loss of gas vacuoles in Halobacterium strain 5 occurs so often that regular recloning of individual colonies containing gas vacuoles is necessary to maintain the wild-type strain⁴. Gas vacuoles are also lost spontaneously in other aquatic microorganisms, and it was this observation which led Walsby to propose that the gas-vacuole protein might be carried on a plasmid⁵. Plasmids have not previously been shown to exist in Halobacterium. All the obligate halophiles as yet examined contain a satellite DNA which may account for between 11 % and 36% of the total DNA (ref. 6, 7). Moore and McCarthy⁷, however, argued that the satellite DNA is not composed of multiple copies of a plasmid because its amount is unchanged by growing the cells in the presence of known plasmid-curing agents, and because total Halobacterium DNA renatures at a rate expected for single-copy DNA. Here I show that the wild-type Halobacterium strain 5 contains three plasmids, and also show a correlation between the presence of a plasmid of molecular weight (MW) 44×10⁶ and the presence of gas vacuoles.