Gas vesicles are unique organelles produced by many aquatic prokaryotic microorganisms and used presumably for flotation. They are bounded by an unusually stable proteinaceous membrane that enfolds a gas filled space. Although detected in numerous groups of prokaryotic species representing both archaea and true bacteria since their discovery in 1888 by Winogradsky, the general morphology of the vesicles appears very similar and at least the major proteins appear homologous. For many years, researchers hypothesized that gas vesicles were formed from a single self-associating protein. However, recent molecular genetic work by DasSarma and his colleagues, using the archaeal prokaryote Halobacterium as their model system, led to the discovery of a large cluster of genes involved in gas vesicle formation. Further, they were able to transfer the gene cluster to laboratory strains of E. coli and engineer the expression of gas vesicles in this bacterium, which does not naturally produce gas vesicles. The gene cluster is hypothesized to comprise three genes coding for structural proteins, in rightward transcriptional orientation, and ten or more genes encoding regulatory or initiation proteins, in leftward transcriptional orientation. DasSarma and his colleagues are continuing their work on biogenesis of gas vesicles in Halobacterium, using immunological and molecular-genetic methods to analyze the sequence and timing of steps in protein formation, assembly, and regulation to build functional gas vesicles. Individual proteins are to be tagged using immunological procedures, and then localized within the organism using immunogold cytological nethods. Mutants strains of Halobacterium will also be studied to determine the sequence of gene activation and action leading to vesicle formation. Complementation studies with mutants and purified protein products will help understand the dynamics of protein interactions in building the vesicle membrane. The overall goal is to describe the component proteins and regulatory signals in the biogenesis of gas vesicles and understand the sequence of genetic and metabolic events leading to their formation
