STRUCTURAL CHARACTERISTICS OF HALOBACTERIAL GAS VESICLES

Gas vesicle formation in halophilic archaea is encoded by a DNA region (the vac region) containing 14 different genes: gvpACNO and gvpDEFGHI JKLM. In Halobacterium salinarum PHH1 (which expresses the p-vac regio n from plasmid pHH1), gas vesicles are spindle shaped, whereas predomi nantly cylindrical gas vesicles are synthesized by the chromosomal c-v ac region of if. salinarum PHH4 and the single chromosomal me-vac regi on of Haloferax mediterranei. Homologous complementation of gvp gene c lusters derived from the chromosomal c-vac region led to cylindrical g as vesicles in transformants and proved that the activity of the c-gvp A promoter depended on a gene product from the c-gvpE-M DNA region. He terologous complementation experiments with transcription units of dif ferent vac regions demonstrated that the formation of chimeric gas ves icles was possible. Comparison of micrographs of wild-type and chimeri c gas vesicles indicated that the shape was not exclusively determined by GvpA, the major structural protein of the gas vesicle wall. More l ikely, a dynamic equilibrium of several gvp gene products was responsi ble for determination of the shape. Transmission electron microscopy o f frozen hydrated, wild-type gas vesicles showed moire patterns due to the superposition of the front and back parts of the ribbed gas vesic le envelope. Comparison of projections of model helices with the moire pattern seen on the cylindrical part of the gas vesicles provided evi dence that the ribs formed a helix of low pitch and not a stack of hoo ps.