Sequence of the ompH gene from the deep-sea bacterium Photobacterium SS9

Molecular basis of bacterial outer membrane permeability revisited

Gram-negative bacteria characteristically are surrounded by an additional membrane layer, the outer membrane. Although outer membrane components often play important roles in the interaction of symbiotic or pathogenic bacteria with their host organisms, the major role of this membrane must usually be to serve as a permeability barrier to prevent the entry of noxious compounds and at the same time to allow the influx of nutrient molecules. This review summarizes the development in the field since our previous review (H. Nikaido and M. Vaara, Microbiol. Rev. 49:1-32, 1985) was published. With the discovery of protein channels, structural knowledge enables us to understand in molecular detail how porins, specific channels, TonB-linked receptors, and other proteins function. We are now beginning to see how the export of large proteins occurs across the outer membrane. With our knowledge of the lipopolysaccharide-phospholipid asymmetric bilayer of the outer membrane, we are finally beginning to understand how this bilayer can retard the entry of lipophilic compounds, owing to our increasing knowledge about the chemistry of lipopolysaccharide from diverse organisms and the way in which lipopolysaccharide structure is modified by environmental conditions.

RNA arbitrarily primed PCR survey of genes regulated by ToxR in the deep-sea bacterium Photobacterium profundum strain SS9

We are currently investigating the role of ToxR-mediated gene regulation in Photobacterium profundum strain SS9. SS9 is a moderately piezophilic ("pressure loving") psychrotolerant marine bacterium belonging to the family Vibrionaceae. In Vibrio cholerae, ToxR is a transmembrane DNA binding protein involved in mediating virulence gene expression in response to various environmental signals. A homolog to V. cholerae ToxR that is necessary for pressure-responsive gene expression of two outer membrane protein-encoding genes was previously found in SS9. To search for additional genes regulated by ToxR in SS9, we have used RNA arbitrarily primed PCR (RAP-PCR) with wild-type and toxR mutant strains of SS9. Seven ToxR-activated transcripts and one ToxR-repressed transcript were identified in this analysis. The cDNAs corresponding to these partial transcripts were cloned and sequenced, and ToxR regulation of their genes was verified. The products of these genes are all predicted to fall into one or both of two functional categories, those whose products alter membrane structure and/or those that are part of a starvation response. The transcript levels of all eight newly identified genes were also characterized as a function of hydrostatic pressure. Various patterns of pressure regulation were observed, indicating that ToxR activation or repression cannot be used to predict the influence of pressure on gene expression in SS9. These results provide further information on the nature of the ToxR regulon in SS9 and indicate that RAP-PCR is a useful approach for the discovery of new genes under the control of global regulatory transcription factors.

The anti-sigma factors

A mechanism for regulating gene expression at the level of transcription utilizes an antagonist of the sigma transcription factor known as the anti-sigma (anti-sigma) factor. The cytoplasmic class of anti-sigma factors has been well characterized. The class includes AsiA form bacteriophage T4, which inhibits Escherichia coli sigma 70; FlgM, present in both gram-positive and gram-negative bacteria, which inhibits the flagella sigma factor sigma 28; SpoIIAB, which inhibits the sporulation-specific sigma factor, sigma F and sigma G, of Bacillus subtilis; RbsW of B. subtilis, which inhibits stress response sigma factor sigma B; and DnaK, a general regulator of the heat shock response, which in bacteria inhibits the heat shock sigma factor sigma 32. In addition to this class of well-characterized cytoplasmic anti-sigma factors, a new class of homologous, inner-membrane-bound anti-sigma factors has recently been discovered in a variety of eubacteria. This new class of anti-sigma factors regulates the expression of so-called extracytoplasmic functions, and hence is known as the ECF subfamily of anti-sigma factors. The range of cell processes regulated by anti-sigma factors is highly varied and includes bacteriophage phage growth, sporulation, stress response, flagellar biosynthesis, pigment production, ion transport, and virulence.

Pressure regulates osteoclast formation and MCSF expression in marrow culture

One of the forces generated during skeletal loading is hydrostatic pressure. In the work presented here, the ability of increased pressure to influence recruitment of osteoclasts was evaluated. Murine marrow cultures, with pO2 and pCO2 kept constant, were subjected to either control (1.0 atm) or elevated (1.37 or 2.0 atm) hydrostatic pressure. As compared to control, cultures pressurized for 6 days at 1.37 atm formed less osteoclast-like cells (OCLC) (71 +/- 6% of control, P < 0.0001). A similar degree of inhibition occurred in cultures exposed to pressure during days 2-4 only (62 +/- 6%), while treatment during days 5-7 failed to inhibit the OCLC number relative to control (99 +/- 5%). Delivery of 2.0 atm pressure on days 2-4 generated 52 +/- 4% OCLC compared to control. Since macrophage colony stimulating factor (MCSF)-dependent proliferation of osteoclast precursors occurs during the pressure-sensitive period, semiquantitative RT-PCR for MCSF mRNA was performed after 3 days in 1.37 atm (days 2-4). As compared to controls, pressure caused a decrease in mRNA coding for the membrane bound form of MCSF (71.2 +/- 4% (n = 25, P < or = 0.05), while the MCSF RT-PCR product representing the secreted form showed no consistent change. This lack of response of the soluble MCSF RT-PCR product was expected, as levels of bioassayable MCSF were not altered by pressure. Extrapolating these data to in vivo conditions suggests that load-bearing will inhibit the formation of osteoclasts.

Isolation and characterization of the structural gene for OmpL, a pressure-regulated porin-like protein from the deep-sea bacterium Photobacterium species strain SS9

Transposon-directed cloning was used to isolate the ompL gene from the deep-sea bacterium Photobacterium species strain SS9. The deduced amino acid sequence of OmpL displays sequence homology to porin proteins from enteric bacteria. Gene fusion and primer extension analyses indicate that ompL is transcriptionally regulated by pressure.

A new gene locus of Bordetella pertussis defines a novel family of prokaryotic transcriptional accessory proteins

Recently, a novel type of regulatory mutation causing differential effects on the expression of virulence genes due to a slight overexpression of the RNA polymerase alpha subunit (RpoA) was found in Bordetella pertussis (N. H. Carbonetti, T. M. Fuchs, A. A. Patamawenu, T. J. Irish, H. Deppisch, and R. Gross, J. Bacteriol. 176:7267-7273, 1994). To gather information on the molecular events behind this phenomenon, we isolated suppressor mutants of the RpoA-overexpressing strains after random mutagenesis. Genetic characterization of these suppressor strains revealed the existence of at least three distinct groups of dominant alleles. Mutations occurred either in the rpoA locus itself, in the bvg locus, or in unknown gene loci. One mutant of the latter group was further characterized. By the introduction of a cosmid library containing genomic B. pertussis DNA into this suppressor strain, we isolated a cosmid which suppressed the phenotype of the suppressor strain, thus restoring the negative effect on transcription of the ptx and cya toxin genes. Mutagenesis of the cosmid with Tn5 led to the identification of the gene locus responsible for this phenomenon. Its DNA sequence revealed the presence of an open reading frame (ORF) consisting of 2,373 bp coding for a hypothetical 86-kDa protein with extensive sequence similarities to ORFs with not yet identified functions of Escherichia coli, Haemophilus influenzae, and Neisseria meningitidis. The new gene, termed tex, for toxin expression, seems to be an essential factor for B. pertussis, as it cannot be deleted from the bacterial chromosome. All members of this new protein family show significant sequence similarities with the mannitol repressor protein MtlR and with the presumptive RNA-binding domains of the Pnp and ribosomal S1 proteins of E. coli in their N- and C-terminal parts, respectively. These sequence similarities and the fact that the tex gene was isolated by virtue of its effects on gene expression in B. pertussis indicate that the members of this new protein family may play an important role in the transcription machinery of prokaryotic organisms.

High pressure influences on gene and protein expression

Elevated hydrostatic pressure can influence gene and protein expression in both 1 atmosphere-adapted and high pressure-adapted microorganisms. Here we review experiments documenting these effects and describe their significance towards understanding the molecular bases of life in deep-sea high pressure environments.

ompH gene expression is regulated by multiple environmental cues in addition to high pressure in the deep-sea bacterium Photobacterium species strain SS9

Photobacterium species strain SS9 is a moderately barophilic (pressure-loving) deep-sea bacterial species which induces the expression of the ompH gene in response to elevated pressure. Here we demonstrate that at 1 atm (1 atm = 1.01325 x 10(5) Pa), ompH expression increases with cell density in 2216 marine medium batch culture and is subject to catabolite repression and the OmpH synthesis is inducible by energy (carbon) starvation. Regulatory mutants which are impaired in ompH gene expression at high pressure are also impaired in cell density regulation of ompH gene expression, indicating that the two inducing conditions overlap in their signal transduction pathways. The same promoter was activated by high cell density at 1 atm of pressure as well as during low-cell-density growth at 272 atm. Catabolite repression of ompH gene expression was induced by a variety of carbon sources, and this repression could be partially reversed in most cases by the addition of cyclic AMP (cAMP). Surprisingly, glucose repression of ompH transcription occurred only at 1 atm, not at 272 atm, despite the fact that catabolite repression was operational in SS9 under both conditions. It is suggested that ompH expression is cAMP and catabolite repressor protein dependent at 1 atm but becomes cAMP and perhaps catabolite repressor protein independent at 272 atm. Possible mechanisms of ompH gene activation are discussed.

Genetic characterization of ompH mutants in the deep-sea bacterium Photobacterium sp. strain SS9

OmpH is an outer membrane protein produced by the deep-sea bacterium Photobacterium species strain SS9 in response to elevated hydrostatic pressure. In order to facilitate studies of the function of this protein, a series of OmpH+ and OmpH- strains were obtained from SS9 by Tn5 gene replacement mutagenesis. A previously isolated ompH::lacZ strain and a derivative of this strain harboring a plasmid expressing the wild-type ompH gene were also utilized. The acridine mutagen ICR 191 preferentially inhibited the growth of OmpH+ over OmpH- cells. Indeed, OmpH+ cultures treated with the mutagen rapidly accumulated mutants producing reduced levels of OmpH. In addition, OmpH+ cells took up the peptide Met-Leu-Phe approximately 15 times more rapidly than OmpH- cells. The results are consistent with the hypothesis that OmpH functions as a relatively large, nonspecific diffusion channel.

Heat shock- and alkaline pH-induced proteins of Campylobacter jejuni: characterization and immunological properties

The protein response to physiological stress was characterized in Campylobacter jejuni 81176 after exposure to heat and pH shock and following periods of recovery. Immunoreactivities of major stress-related proteins were determined with anti-Campylobacter immune rabbit serum and intestinal lavage fluid. Distinct proteins with molecular masses ranging from 10 to 120 kDa were induced and/or released by selective heat or pH treatments. The most notable responses were those of two proteins with apparent molecular masses of 45 and 64 kDa that were induced and two other proteins of 10 and 12 kDa that were released by selective heat shock, alkaline pH treatment, or both. On the basis of N-terminal sequence analysis and immunological cross-reactivity data, the 64- and 10-kDa proteins were the C. jejuni homologs of Escherichia coli GroEL and GroES proteins, respectively. Enhanced chemiluminescence Western blotting (immunoblotting) revealed that all four proteins were among the major protein antigens recognized by anti-Campylobacter rabbit serum immunoglobulin G (IgG) and immune rabbit intestinal lavage IgA (secretory IgA). The results of this investigation suggest that the C. jejuni 10-, 12-, 45-, and 64-kDa proteins and a number of minor stress-related proteins deserve further evaluation of their respective roles in Campylobacter pathogenesis and immunity.

High pressure conditions stimulate expression of chloramphenicol acetyltransferase regulated by the lac promoter in Escherichia coli

Recombinant plasmids with the chloramphenicol acetyltransferase (CAT) structural gene behind several kinds of promoters were tested for expression in Escherichia coli during growth at atmospheric pressure (0.1 MPa) and at high pressure (30 MPa). Expression of the CAT gene from the lac promoter was remarkably activated (approx. 78-fold) by high pressure in the absence of the inducer isopropyl-beta-D-thiogalactopyranoside (IPTG). The stimulation of the CAT activity by the lac promoter at high pressure did not simply result from an increased plasmid copy number, because the CAT activities from the other promoters and beta-lactamase activities were unaffected at high pressure.

Use of a reporter gene to follow high-pressure signal transduction in the deep-sea bacterium Photobacterium sp. strain SS9

Photobacterium sp. strain SS9 is a deep-sea bacterium which modulates the abundances of several outer membrane proteins as a function of hydrostatic pressure. These proteins include the product of the previously cloned ompH gene (D. H. Bartlett, M. Wright, A. A. Yayanos, and M. Silverman. Nature (London) 342:572-574, 1989). Subsequent to conjugal plasmid delivery it was possible to cross an ompH::lacZ transcriptional fusion into the genome of SS9, replacing the wild-type ompH gene, generating strain EC10. EC10 is not impaired in growth at high pressure, indicating that under the growth conditions employed, OmpH is not required for baroadaptation. beta-Galactosidase production in EC10 is induced by high pressure to approximately the same extent that OmpH production is in the parental strain, SS9. Therefore, OmpH abundance appears to be primarily regulated at the transcriptional level. EC10 was used for the isolation of ompH regulatory mutants. Derivatives of EC10 which produce reduced levels of beta-galactosidase at both low and high pressure and which appeared to possess mutations outside the ompH::lacZ locus were obtained. All of these regulatory mutants displayed alterations in the high-pressure repression of a second outer membrane protein, designated OmpL, and two of the mutants were also deficient in the high-pressure induction of a third outer membrane protein, designated OmpI. The most dramatic phenotype was present in mutant EC1002, whose growth was extremely barosensitive. EC1002 is the first pressure-sensitive mutant ever isolated. Prolonged incubation of EC1002 at high pressure led to the accumulation of cells with wild-type growth characteristics at high pressure. These cells are suggested to possess suppressor mutations, as they remain deficient in beta-galactosidase production and maintain their high-pressure-adapted phenotype for many generations in the absence of high-pressure selection.