Expression of the major gas vesicle protein gene in the halophilic archaebacteriumHaloferax mediterranei is modulated by salt

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Abstract
In the moderately to extremely halophilic archaebacteriumHaloferax mediterranei gas vacuoles are not observed before the stationary phase of growth, and only when the cells are grown in media containing more than 17% total salt. Under the electron microscope, isolated gas vesicles appear as cylindrical structures with conical ends that reach a maximal length of 1.5 μm; this morphology is different from the spindle-shaped gas vesicles found in theHalobacterium halobium wild type which expresses the plasmid-borne p-vac gene, but resembles that of gas vesicles isolated fromH. halobium strains expressing the chromosomalc-vac gene. Both the p-vac and thec-vac genes encode very similar structural proteins accounting for the major part of the “membrane” of the respective gas vesicles. The homologousmc-vac gene was isolated fromHf. mediterranei using thep-vac gene as probe. Themc-vac coding region indicates numerous nucleotide differences compared to thep-vac ancc-vac genes; the encoded protein is, however, almost identical to thec-vac gene product. The start point of the 310 nucleotidemc-vac transcript determined by primer extension analysis and S1 mapping was located 20 by upstream of the ATG start codon, which is at the same relative position as found for the other twovac mRNAs. During the growth cycle,mc-vac mRNA was detectable inHf. mediterranei cells grown in 15% as well as 25% total salt, with a maximal level in the early stationary phase of growth. The relative abundance ofmc-vac mRNA in cells grown at 25% salt was sevenfold higher than in cells grown in 15% total salt.