Chemical composition of gas vesicles isolated from Anabaena flos-aquae

Direct measurements of steady-state kinetics of cyanobacterial n(2) uptake by membrane-leak mass spectrometry and comparisons between nitrogen fixation and acetylene reduction

A mass spectrometer with a membrane-covered inlet was used to measure nitrogen fixation by following changes in the concentration of dissolved N(2) in a stirred suspension of the cyanobacterium Anabaena variabilis in an open system. The results showed a good fit to Michaelis-Menten kinetics with a K(m) for N(2) of 65 muM at 35 degrees C, corresponding to 0.121 atmosphere of N(2). Corresponding values for the K(m) for acetylene reduction were 385 muM (0.011 atmosphere at 35 degrees C). Comparison of the values of V(max) for N(2) uptake with those for the acetylene reduction assay under similar conditions gave an average value of 3.8 for the conversion factor between N(2) and C(2)H(2) reduction. Reduction of protons to hydrogen was completely inhibited at sufficiently high concentrations of C(2)H(2), but even at saturating N(2) concentrations, 1 mol of H(2) was produced for every mole of N(2) reduced. This explains the finding that the observed C(2)H(2)/N(2) ratio is higher than the value of 3 expected from the requirement for two electrons for acetylene reduction and six for nitrogen reduction. The results correlate well with a mechanism for N(2) reduction involving the equation: N(2) + 8H + 8e --> 2NH(3) + H(2) which gives a conversion factor between C(2)H(2) and N(2) of 4. It is proposed that, in general, 4 is a more appropriate value than 3 for the conversion factor.

Lower limit of the gas permeability coefficient of gas vesicles

The gas vesicles found in various planktonic prokaryotes are hollow, rigid structures permeable to gases. They collapse when the difference between the external hydrostatic pressure and internal gas pressure exceeds their critical pressure (usually about 0.6 MPa). It was found that dried gas vesicles would survive exposure to gas pressures considerably in excess of this value (4 MPa or more), because gas diffused into them as the pressure was raised and the pressure difference required to cause collapse was not established. They survived the most rapid rates of pressure rise, 0–4.6 MPa in less than 2.5 ms, to which they were exposed. From this it can be calculated that the gas permeability coefficient of the average gas vesicle ( α ) exceeds 22 x 10 3 s -1 and the permeability of the gas vesicle wall ( k ) is greater than 332 μm s -1 . Gas molecules may diffuse through fixed pores in the gas vesicle wall. Since a gas molecule of collision diameter 0.63 nm is known to penetrate the gas vesicle, this would be the minimum diameter of such a fixed pore. It is shown by kinetic theory that the permeability coefficient of an average gas vesicle with one pore of this size would be 2.1 x 10 3 s -1 : there would, therefore, have to be at least 11 such pores to account for the observed minimum permeability coefficient. Gas vesicles in aqueous suspension will also survive a rapid rise in the overlying gas pressure in excess of their critical pressure if they are near enough to the gas-water interface for sufficient gas to reach them by diffusion during the pressure rise. The distance from the interface at which the gas vesicles survive can be used to calculate the diffusivity of the gas through the suspension. A modification of this method can be used to measure the gas-permeability of certain types of cells containing gas vesicles.

Gas vesicles

The gas vesicle is a hollow structure made of protein. It usually has the form of a cylindrical tube closed by conical end caps. Gas vesicles occur in five phyla of the Bacteria and two groups of the Archaea, but they are mostly restricted to planktonic microorganisms, in which they provide buoyancy. By regulating their relative gas vesicle content aquatic microbes are able to perform vertical migrations. In slowly growing organisms such movements are made more efficiently than by swimming with flagella. The gas vesicle is impermeable to liquid water, but it is highly permeable to gases and is normally filled with air. It is a rigid structure of low compressibility, but it collapses flat under a certain critical pressure and buoyancy is then lost. Gas vesicles in different organisms vary in width, from 45 to > 200 nm; in accordance with engineering principles the narrower ones are stronger (have higher critical pressures) than wide ones, but they contain less gas space per wall volume and are therefore less efficient at providing buoyancy. A survey of gas-vacuolate cyanobacteria reveals that there has been natural selection for gas vesicles of the maximum width permitted by the pressure encountered in the natural environment, which is mainly determined by cell turgor pressure and water depth. Gas vesicle width is genetically determined, perhaps through the amino acid sequence of one of the constituent proteins. Up to 14 genes have been implicated in gas vesicle production, but so far the products of only two have been shown to be present in the gas vesicle: GvpA makes the ribs that form the structure, and GvpC binds to the outside of the ribs and stiffens the structure against collapse. The evolution of the gas vesicle is discussed in relation to the homologies of these proteins.

Gas vesicle synthesis in the cyanobacterium Pseudanabaena sp.: occurrence of a single photoregulated gene

Gas vesicles are subcellular inclusions found in a large number of aquatic prokaryotes. The gvpA gene, which frequently occurs as a multigene family, encodes the major gas vesicle structural protein. In several cyanobacteria, another gene, gvpC, encodes a different protein which might be a dispensable element for gas vesicle formation. We report here the molecular characterization of a gvpA gene in Pseudanabaena sp. PCC 6901. In this planktonic cyanobacterium, it is the only gvp gene which could be detected, and electrophoretic analysis of isolated gas vesicles revealed the presence of a single protein. A monocistronic mRNA species corresponds to the transcription of the gvpA gene and the abundance of the gvpA mRNA is inversely correlated with photosynthetic photon flux indicating that a light-dependent transcriptional regulation is likely to be involved in the control of gas vacuolation in this strain.

Gas vesicle proteins

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Occurrence and distribution of gas vesicle genes among cyanobacteria

Gas vesicles (GV) are specialized cell inclusions providing many aquatic procaryotes with buoyancy. In the cyanobacterium Calothrix sp. strain PCC 7601, at least four genes are involved in GV formation. One of those, gvpA1, encodes the major structural GV protein (70 amino acids) and belongs to a multigene family (gvpA1, gvpA2, gvpD). The fourth gene, gvpC, encodes a 162-amino-acid protein, the function of which is still unclear. We used the Calothrix gvpA1 and gvpC genes as probes to perform Southern hybridization experiments with DNA extracted from various cyanobacterial strains. The gvpA gene was found in all the strains that synthesize GV, indicating that its product is an obligatory component of GV. Furthermore, it was found to occur as multiple copies in most of the strains tested. The gvpC gene was only detected in some strains able to synthesize a large amount of GV within a short period. This suggests that the gvpC gene product is a dispensable protein for GV formation and is involved in the efficiency of the assembly process. Based on the occurrence of the gvp genes and on DNA-DNA hybridization patterns, genus assignments are discussed.

Molecular cloning and nucleotide sequence of a developmentally regulated gene from the cyanobacterium Calothrix PCC 7601: a gas vesicle protein gene

Since the gas vesicle protein (GVP) is highly conserved among the different gas-vacuolate prokaryotes, a 29-mer oligonucleotide corresponding to a portion of the Anabaena flos-aquae GVP gene was synthesized and used to isolate the GVP structural gene from Calothrix PCC 7601 (= Fremyella diplosiphon). Gas vacuole production in this filamentous cyanobacterium is restricted to hormogonia which occur at a specific stage during the developmental cell cycle. The GVP gene (gvpA) was localized on a 709 bp HindIII-HincII fragment. Nucleotide sequence analysis revealed a 213 bp open reading frame whose deduced amino-acid sequence shows a very high homology with that of the Anabaena flos-aquae GVP. Assuming that the first methionine residue is proteolytically processed, the molecular mass of the Calothrix GVP is 7375 daltons. Sequences resembling the Escherichia coli consensus promoter were found upstream from the gvpA gene. The initiator codon of the gvpA gene is preceded by a polypurine sequence assumed to be the ribosome binding site. Southern hybridizations with a probe specific for the gvpA gene indicated that this gene is not plasmid-borne, and that another homologous gene is present in the Calothrix genome.

Molecular weight of gas-vesicle protein from the planktonic cyanobacterium Anabaena flos-aquae and implications for structure of the vesicle

The gas vesicle of the planktonic cyanobacterium Anabaena flos-aquae is a cylindrical shell made of protein enclosing a gas-filled space. Protein sequence analysis shows that the vesicle is made from a single protein. By gel electrophoresis and amino acid analysis its molecular weight was estimated to be 20 600. Taken with previously obtained X-ray data, a simple interpretation of its molecular structure is of the polypeptide snaking in six pairs of antiparallel chains, three in each layer. The molecule would repeat along the ribs of the vesicle at intervals of 3.4 nm.

Isolation and characterization of gas vesicles from Microcyclus aquaticus

Intact gas vesicles of Microcyclus aquaticus S1 were isolated by using centrifugally accelerated flotation of vesicles and molecular sieve chromatography. Isolated gas vesicles were cylindrical organelles with biconical ends and measured 250x100nm. The gas vesicle membrane was composed almost entirely of protein; neither lipid nor carbohydrate was detected, although one mole of phosphate per mole of protein was found. Amino acid analysis indicated that the protein contained 54.6% hydrophobic amino acid residues, lacked sulfur-containing amino acids, and had a low aromatic amino acid content. The protein subunit composition of the vesicles was determined by gel electrophoresis in (i) 0.1% sodium dodecyl sulfate at pH 9.0 and (ii) 5 M urea at pH 2.0. The membrane appeared to consist of one protein subunit of MW 50000 daltons. Charge isomers of this subunit were not detected on urea gels. Antiserum prepared against purified gas vesicles of M. aquaticus S1 crossreacted with the gas vesicles of all other gas vacuolate strains of M. aquaticus, as well as those of Prosthecomicrobium pneumaticum, Nostoc muscorum, and Anabaena flos-aquae, indicating that the gas vesicles of these widely divergent organisms have some antigenic determinants in common.