The use of transposon TnphoA to detect genes for cell envelope proteins subject to a common regulatory stimulus. Analysis of osmotically regulated genes in Escherichia coli

Trehalose synthesis genes are controlled by the putative sigma factor encoded by rpoS and are involved in stationary-phase thermotolerance in Escherichia coli

The rpoS (katF) gene of Escherichia coli encodes a putative sigma factor (sigma S) required for the expression of a variety of stationary phase-induced genes, for the development of stationary-phase stress resistance, and for long-term starvation survival (R. Lange and R. Hengge-Aronis, Mol. Microbiol. 5:49-59, 1991). Here we show that the genes otsA, otsB, treA, and osmB, previously known to be osmotically regulated, are also induced during transition into stationary phase in a sigma S-dependent manner. otsA and otsB, which encode trehalose-6-phosphate synthase and trehalose-6-phosphate phosphatase, respectively, are involved in sigma S-dependent stationary-phase thermotolerance. Neither sigma S nor trehalose, however, is required for the development of adaptive thermotolerance in growing cells, which might be controlled by sigma E.

Neisseria gonorrhoeae prepilin export studied in Escherichia coli

The pilE gene of Neisseria gonorrhoeae MS11 and a series of pilE-phoA gene fusions were expressed in Escherichia coli. The PhoA hybrid proteins were shown to be located in the membrane fraction of the cells, and the prepilin product of the pilE gene was shown to be located exclusively in the cytoplasmic membrane. Analysis of the prepilin-PhoA hybrids showed that the first 20 residues of prepilin can function as an efficient export (signal) sequence. This segment of prepilin includes an unbroken sequence of 8 hydrophobic or neutral residues that form the N-terminal half of a 16-residue hydrophobic region of prepilin. Neither prepilin nor the prepilin-PhoA hybrids were processed by E. coli leader peptidase despite the presence of two consensus cleavage sites for this enzyme just after this hydrophobic region. Comparisons of the specific molecular activities of the four prepilin-PhoA hybrids and analysis of their susceptibility to proteolysis by trypsin and proteinase K in spheroplasts allow us to propose two models for the topology of prepilin in the E. coli cytoplasmic membrane. The bulk of the evidence supports the simplest of the two models, in which prepilin is anchored in the membrane solely by the N-terminal hydrophobic domain, with the extreme N terminus facing the cytoplasm and the longer C terminus facing the periplasm.

LcrD, a membrane-bound regulator of the Yersinia pestis low-calcium response

Yersinia pestis, the etiologic agent of bubonic plague, contains a 75-kb virulence plasmid, called pCD1 in Y. pestis KIM. The low-Ca(2+)-response genes of Y. pestis regulate both bacterial growth and the expression of pCD1-encoded virulence determinants in response to temperature and the presence of Ca2+ or nucleotides. This study characterizes the nucleotide sequence and protein product of the lcrD locus. An lcrD mutant, in contrast to the parent Y. pestis, did not undergo growth restriction or induce strong expression of the V antigen when grown under conditions (37 degrees C, no Ca2+) expected to elicit maximal expression of pCD1 genes. DNA sequence analysis of the cloned lcrD locus showed a single open reading frame that could encode a protein with a molecular weight of 77,804 and a pI of 4.88. LcrD was identified as a 70-kDa inner membrane protein by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblot analysis. LcrD membrane topology was investigated by using lcrD-phoA translational fusions generated with the transposon TnphoA. The alkaline phosphatase activities of the resultant hybrid proteins were consistent with a model predicting eight amino-terminal transmembrane segments that anchor a large cytoplasmic carboxyl-terminal domain to the inner membrane.

Identification of Escherichia coli genes whose expression increases as a function of external pH

Using transposon TnphoA and a plate screening method, we have isolated a set of Escherichia coli strains carrying phoA fusions with genes whose expression is modulated as a function of external pH. Besides fusions with the ompF gene and the malB locus, thirteen independent fusions were analysed whose expression is maximal during growth at pHs ranging from 7.0 to 8.5 and minimal during growth at pH 5.0. Six different genetic loci, called phmA, phmB, phmC, phmD, phmE and phmF (for pH modulated) were characterized and localized on the E. coli chromosome at approx. 12, 18, 41, 45, 75 and 84 min, respectively. Expression of phmA::phoA fusions is also influenced when internal pH or environmental conditions such as osmolarity or anaerobiosis are modified. EnvZ protein is not involved in the regulation of phm::phoA fusions.

Osmotic induction of gene osmC expression in Escherichia coli K12

osmC, an osmotically inducible gene of Escherichia coli, was physically mapped on the bacterial chromosome, cloned on multicopy plasmids, and its product, OsmC, was identified as a 14 kDa protein in maxicells. The DNA sequence of the gene and its upstream region were determined. The sequence of an osmC-phoA gene fusion confirmed the osmC reading frame. A deletion of osmC from the E. coli chromosome was constructed by gene replacement, demonstrating that it is not an essential gene. The osmCp promoter region was subcloned and a lac operon fusion transcribed under osmCp control was constructed. The expression of this operon fusion demonstrated that osmC regulation occurs at the transcriptional level. S1 nuclease protection experiments and deletion analysis identified two overlapping promoters with transcription start sites separated by ten nucleotides. All the sequences necessary for osmotic regulation of both promoters are located within a 137 base-pair DNA fragment extending from position -95 to +42 with respect to the putative osmC translation start. Two deletions were obtained that abolish the functioning of the upstream promoter. Yet, under our experimental conditions, the subsequent expression of the osmC-lacZ fusion was equivalent to that obtained from the tandem promoters. Mutations leading to constitutive expression of osmC were selected. Two independent mutations were obtained, both affected osmZ, the gene encoding the histone-like protein H1.

Localization and orientation of the VirD4 protein of Agrobacterium tumefaciens in the cell membrane

The virD4 gene of Agrobacterium tumefaciens is essential for the formation of crown galls. Analysis of the nucleotide sequence of virD4 has suggested that the N-terminal region of the encoded protein acts as a signal peptide for the transport of the VirD4 protein to the cell membrane of Agrobacterium. We have examined the localization and orientation of this protein in the cell membrane. When the nucleotides encoding the first 30 to 41 amino acids from the N-terminus of the VirD4 protein were fused to the gene for alkaline phosphatase from which the signal sequence had been removed, alkaline phosphatase activity was detectable under appropriate conditions. Immunoblotting with VirD4-specific antiserum indicated that the VirD4 protein could be recovered exclusively from the membrane fraction of Agrobacterium cells. Moreover, when the membrane fraction was separated into inner and outer membrane fractions by sucrose density-gradient centrifugation, VirD4 protein was detected in the inner-membrane fraction and in fractions that sedimented between the inner and outer membrane fractions. By contrast, the VirD4'/alkaline phosphatase fusion protein with the N-terminal sequence from VirD4 was detected only in the inner membrane fraction. Treatment of spheroplasts of Agrobacterium cells with proteinase K resulted in digestion of the VirD4 protein. These results indicate that the VirD4 protein is transported to the bacterial membrane and anchored on the inner membrane by its N-terminal region. In addition, the C-terminal portion of the VirD4 protein probably protrudes into the periplasmic space, perhaps in association with some unidentified cellular factor(s).

Outer membrane protein A (OmpA) contributes to serum resistance and pathogenicity of Escherichia coli K-1

We examined whether outer membrane protein A (OmpA) contributes to gram-negative pathogenesis by determining the effect of mutagenesis of ompA in a virulent Escherichia coli K-1 isolate. An OmpA mutant was generated by insertion of the transposon TnphoA, which was genetically modified to increase the efficiency of its delivery by conjugation. The mutant was less virulent than its parent strain in two models of E. coli K-1 infection. Equal inocula of the OmpA+ and OmpA- strains fed to neonatal rats resulted in a sevenfold-greater incidence of bacteremia at 72 h from the OmpA+ strain. The lethal effect of the OmpA- mutant was significantly less than that of the OmpA+ parent strain when inoculated onto the chorioallantoic membrane of 10-day embryonated chick eggs. There was, however, no difference between strains in growth characteristics under physiologic conditions, either in rat serum or in unembryonated chick eggs. In the presence of a 10-day chick embryo, there was a 10-fold increase in the survival and growth of the OmpA+ strain. Correction of the mutation in ompA with an E. coli K-12 ompA gene restored a level of virulence equivalent to that of the parent strain. The ompA mutant was more sensitive to the bactericidal effect of pooled human serum by the classical pathway of complement activation. These results suggest that OmpA contributes to E. coli K-1 pathogenesis by a mechanism which may involve increased serum resistance.

The mdoA locus of Escherichia coli consists of an operon under osmotic control

In Escherichia coli, the 5 kb mdoA locus is involved in the osmotically controlled biosynthesis of periplasmic membrane-derived oligosaccharides (MDOs). The structure of this locus was analysed by in vitro cassette insertion, transposon mutagenesis, and gene-fusion analysis. A 'neo' cassette, derived from the neomycin phosphotransferase II region of transposon Tn5, was inserted into mdoA, borne by a multicopy plasmid. This plasmid was shown to complement two previously described mdoA mutations, depending on the orientation of the exogenous gene. Thus, the gene altered by these mutations could be expressed under the control of the exogenous promoter. Moreover, the 'neo' cassette inactivated another, uncharacterized, mdo gene, because when this insertion was transferred into the chromosome MDO synthesis was abolished. The existence of a second gene was confirmed by complementation analysis with a collection of Tn1000 insertions into mdoA. Two groups were defined, and the two genes are organized into an operon (mdoGH). This conclusion was reached because Tn1000 insertions in the first gene displayed a polar effect on the expression of the second gene. An active gene fusion was obtained on a multicopy plasmid between the beginning of mdoH and lacZ. The hybrid beta-galactosidase activity followed the same osmotically controlled response as that described for of MDO synthesis. This regulation was unaffected by the presence, or absence, of MDOs in the periplasm. Finally, the amount of mdoA-specific mRNAs, determined by dot blot hybridization, decreased when the osmolarity of the growth medium increased.

Cloning, nucleotide sequence, and enzymatic characterization of an alpha-amylase from the ruminal bacterium Butyrivibrio fibrisolvens H17c

A Butyrivibrio fibrisolvens amylase gene was cloned and expressed by using its own promoter on the recombinant plasmid pBAMY100 in Escherichia coli. The amylase gene consisted of an open reading frame of 2,931 bp encoding a protein of 976 amino acids with a calculated Mr of 106,964. In E. coli(pBAMY100), more than 86% of the active amylase was located in the periplasm, and TnphoA fusion experiments showed that the enzyme had a functional signal peptide. The B. fibrisolvens amylase is a calcium metalloenzyme, and three conserved putative calcium-binding residues were identified. The amylase showed high sequence homology with other alpha-amylases in the three highly conserved regions which constitute the active centers. These and other conserved regions were located in the N-terminal half, and no similarity with any other amylase was detected in the remainder of the protein. Deletion of approximately 40% of the C-terminal portion of the amylase did not result in loss of amylolytic activity. The B. fibrisolvens amylase was identified as an endo-alpha-amylase by hydrolysis of the Phadebas amylase substrate, hydrolysis of gamma-cyclodextrin to maltotriose, maltose, and glucose and the characteristic shape of the blue value and reducing sugar curves. Maltotriose was the major initial hydrolysis product from starch, although extended incubation resulted in its hydrolysis to maltose and glucose.

Nucleotide sequence of the Escherichia coli regulatory gene mprA and construction and characterization of mprA-deficient mutants

In high copy number, the Escherichia coli mprA gene reduces the synthesis of peptide microcins B17 and C7 (MccB17 and MccC7) and blocks the osmoinduction of the proU operon at the transcriptional level. mprA has been sequenced and shown to encode a polypeptide of 176 amino acids (Mr, 20,563). Insertion and deletion mutant mprA alleles were constructed and then transferred to the chromosome by allelic replacement. In these mutants, expression of two mcb-lacZ fusions was fivefold derepressed, indicating a negative regulatory role of mprA on the mcb operon (MccB17). In contrast, no effect of the MprA- mutations on the expression of mcc operon (MccC7) or on the osmoinduction of proU operon was observed.

Nucleotide sequence and overexpression of the tellurite-resistance determinant from the IncHII plasmid pHH1508a

The transcription and translation of the tellurite-resistance (TeR) genes of the HII incompatibility group plasmid, pHH1508a, were studied. The nucleotide (nt) sequence of the TeR region was determined and two possible open reading frames, tehA and tehB, were identified. The direction of transcription and translation of these genes was confirmed through the preparation of lacZ and phoA (encoding alkaline phosphatase) fusions. The transcription start point was identified in the sequence using RNA primer extension. The tehA gene codes for a 36-kDa polypeptide which is highly hydrophobic. The TehA protein appears to be located in the inner membrane of the bacterial cell since tehA fusions with both phoA and lacZ were obtained and expressed. The tehB gene codes for a 23-kDa polypeptide which appears to be relatively hydrophilic and is probably located in the cytoplasm. Both proteins were overproduced using a T7 RNA polymerase/promoter system. No nt or amino acid sequence homology could be found between this TeR determinant and the TeR genes from the IncHI-2 plasmid, pMER610, and the IncP alpha plasmid, RK2.

Molecular cloning and characterization of a 35.5-kilodalton lipoprotein of Treponema pallidum

A clone expressing a 35.5-kDa recombinant treponemal protein was isolated from a genomic DNA library constructed from Treponema pallidum street strain 14. Polyclonal antiserum raised against the recombinant protein reacted with a corresponding native protein of comparable size in T. pallidum that is specific to the pathogenic treponemes. Radiolabeling of the recombinant protein with [3H]palmitate demonstrated that it is lipid modified. Like other recently characterized T. pallidum lipoproteins, the 35.5-kDa lipoprotein partitioned into the detergent phase from T. pallidum cells fractionated with Triton X-114, suggesting that it is an integral membrane protein. Processing of the recombinant 35.5-kDa lipoprotein from a precursor form to a smaller mature form was not evident in pulse-chase experiments. However, pretreatment of Escherichia coli cells expressing the 35.5-kDa lipoprotein with inhibitors of protein processing or translocation revealed the existence of a higher-molecular-mass precursor. Gene fusion studies with the transposon TnphoA demonstrated the presence of an export signal in the 35.5-kDa lipoprotein that promotes the extracytoplasmic localization of a 35.5-kDa lipoprotein-PhoA hybrid.

Sequence and TnphoA analysis of a Mycoplasma hyorhinis protein with membrane export function

Proteins translocated across the single plasma membrane of mycoplasmas (class Mollicutes) represent important components likely to affect several interactions of these wall-less microbes with their respective hosts. However, identification and functional analysis of such proteins is hampered by the lack of mutational systems in mycoplasmas and by a perceived limitation in translating recombinant mycoplasma genes containing UGA (Trp) codons in other eubacteria. Here we directly analyze a gene encoding a Mycoplasma hyorhinis protein capable of promoting its membrane translocation. It was initially detected by screening a recombinant phage genomic library with antibody from a host with M. hyorhinis-induced arthritis and was localized by Tn5 and deletion mutations affecting expression of antigenic translational products. Sequence analysis of the isolated gene predicted a hydrophilic protein, P101, containing three UGA codons and a putative signal peptide with an uncharacteristic cluster of positively charged amino acids near its C terminus. Nevertheless, lambda::TnphoA transposon mutagenesis of an Escherichia coli plasmid bearing the p101 gene resulted in p101::TnphoA fusions expressing products that could translocate as much as 48 kDa of the P101 sequence (up to the first UGA codon) across the E. coli plasma membrane. Fusion proteins containing mature P101 sequences expressed mycoplasma epitopes and were found by cell fractionation and detergent phase partitioning to be integral membrane proteins in E. coli, suggesting a lack of signal peptide cleavage in this system. Importantly, identification of P101 by direct analysis of its export function relied neither on prior identification of the mycoplasmal product nor on complete expression of the product from the cloned mycoplasma gene.

Osmotic induction of the periplasmic trehalase in Escherichia coli K12: characterization of the treA gene promoter

The Escherichia coli treA gene encodes an osmotically inducible periplasmic trehalase. A strain carrying a treA-lacZ transcriptional fusion was constructed. The beta-galactosidase activity produced in this strain growing exponentially in a medium of high osmotic pressure was 10-fold higher than that produced in a medium of low osmotic pressure, demonstrating that treA transcription is osmotically inducible. treA transcriptional induction depends neither on the presence of trehalase itself nor on the synthesis of cytoplasmic trehalose which occurs in response to osmotic stress in wild-type E. coli strains. The treA promoter was identified by S1 nuclease protection. Deletion analysis demonstrated that sequences sufficient for the osmotic induction lie downstream from nucleotide -40 with respect to the transcription start. Transcription initiation at treAp required the presence of a functional sigma 70 subunit of RNA polymerase. treA expression was increased in the presence of a mutation in osmZ, which was previously identified as leading to a partially constitutive expression of the osmotically inducible proU operon.

Metronidazole activation and isolation of Clostridium acetobutylicum electron transport genes

An Escherichia coli F19 recA, nitrate reductase-deficient mutant was constructed by transposon mutagenesis and shown to be resistant to metronidazole. This mutant was a most suitable host for the isolation of Clostridium acetobutylicum genes on recombinant plasmids, which activated metronidazole and rendered the E. coli F19 strain sensitive to metronidazole. Twenty-five E. coli F19 clones containing different recombinant plasmids were isolated and classified into five groups on the basis of their sensitivity to metronidazole. The clones were tested for nitrate reductase, pyruvate-ferredoxin oxidoreductase, and hydrogenase activities. DNA hybridization and restriction endonuclease mapping revealed that four of the C. acetobutylicum insert DNA fragments on recombinant plasmids were linked in an 11.1-kb chromosomal fragment. DNA sequencing and amino acid homology studies indicated that this DNA fragment contained a flavodoxin gene which encoded a protein of 160 amino acids that activated metronidazole and made the E. coli F19 mutant very sensitive to metronidazole. The flavodoxin and hydrogenase genes which are involved in electron transfer systems were linked on the 11.1-kb DNA fragment from C. acetobutylicum.

Transcriptional analysis, translational analysis, and sequence of the kilA-tellurite resistance region of plasmid RK2Ter

The tellurite resistance (Ter) determinant of the IncP alpha plasmid RK2Ter, a variant of RK2 (also called RP4), is located between the kilA and korA genes involved in plasmid replication control. Transcriptional and translational fusions were constructed between the gene for beta-galactosidase and the kilA and Ter genes by using the transpositional phage mini-Mu. These fusions indicated that the Ter genes are transcribed in the same direction as kilA and that transcription and translation of the cloned kilA gene are occurring and may not be lethal to the bacterial cell even in the absence of korA. The nucleotide sequence of this region was determined, and three open reading frames (ORFs) were identified. The first ORF codes for KilA, a 28-kDa hydrophilic protein. The second ORF, telA, codes for a hydrophilic protein of 42 kDa. The third ORF, telB, codes for a hydrophobic protein of 32 kDa. This protein appears to be located in the inner membrane of the bacterial cell, since fusions of TelB to alkaline phosphatase were obtained by using TnphoA. All three proteins were detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis after overproduction using the T7 RNA polymerase/promoter system. The same three proteins were produced when Tes and Ter derivatives of RP4 were expressed in an in vitro transcription-translation system. A single Ser-to-Cys missense mutation in telB was found to be responsible for mutation of RK2 to Ter.

Characterization of mutations affecting the osmoregulated proU promoter of Escherichia coli and identification of 5' sequences required for high-level expression

Expression of the Escherichia coli proU operon, which encodes an efficient uptake system for the osmoprotectant glycine betaine, is strongly increased in cells grown at high osmolarity. We isolated 182 independent spontaneous mutants with elevated expression of the chromosomal phi(proV-lacZ) (Hyb2) fusion at low osmolarity. Genetic analysis demonstrated that eight of these mutant strains carried mutations closely linked to the fusion, whereas all others carried mutations that appeared to be in osmZ. All of the mutations resulted in increased but still osmoregulated expression of the phi(proV-lacZ)(Hyb2) fusion. The proU-linked mutants carried an identical point mutation (proU603) which changes the -35 sequence of the proU promoter from TTGCCT to TTGACT and thereby increases the homology of the -35 region to the consensus sequence (TTGACA) of E. coli promoters. We also selected for mutants with decreased expression of the plasmid pOS7-encoded phi(proV-lacZ)(Hyb2) fusion and isolated a plasmid with an IS1 insertion (proU607) between the proU -10 and -35 regions. This insertion creates a hybrid promoter and drastically reduces expression of the fusion but does not abolish its osmotic regulation. Deletion analysis of chromosomal sequences 5' to the proU promoter revealed that sequences located approximately 200 bp upstream of the -35 region were required for high-level expression. Removal of these sequences resulted in a 10-fold decline of phi(proV-lacZ)(Hyb2) expression. Osmotic regulation was retained in deletion constructs carrying just 19 bp of chromosomal DNA 5' of the promoter, showing that no sequences further upstream are required for the proper osmoregulation of proU transcription. Experiments with himA and fis mutant strains indicated that the IHF and FIS proteins are not required for the normal osmoregulation of proU expression.

987P fimbrial gene identification and protein characterization by T7 RNA polymerase-induced transcription and TnphoA mutagenesis

The 987P fimbrial gene cluster has been previously cloned as a 12 kb fragment from prototype strain 987. Gene products encoded by the whole clone were analysed by utilizing an in vivo system based on the induction of transcription by T7 RNA polymerase. The sensitivity of this technique permitted us to identify new proteins involved in 987P fimbriation. In total, eight proteins were detected, their genes (fasA to fasH) were mapped and their orientation of transcription determined. Several of the gene products demonstrated typical properties of exported proteins. Precursor and processed forms could be correlated after inhibiting protein transport with ethanol. The detection of enzymatically active fusion proteins after TnphoA (Tn5IS50L::phoA) mutagenesis supported and complemented these results. One protein encoded by the 12kb fragment was found not to be related to fimbriation but rather the product of the STla gene, identified as a component of a Tn1681-like transposon.

Osmotic signal transduction to proU is independent of DNA supercoiling in Escherichia coli

proU expression has been proposed to form part of a general stress response that is regulated by increased negative DNA supercoiling brought about by environmental signals such as osmotic or anaerobic stress (N. Ni Bhriain, C. J. Dorman, and C. F. Higgins, Mol. Microbiol. 3:933-944, 1989). However, we find that although proU-containing plasmids derived from cells grown in media of elevated osmolarity were more supercoiled than plasmids from cells grown in standard media, they did not activate proU expression in vitro. The gyrA96 mutation and anaerobic conditions are known to affect DNA supercoiling but did not alter proU expression. Finally, the gyrase inhibitors coumermycin and novobiocin did not reduce in vitro proU expression. Therefore, this evidence rules out regulation by changes in DNA superhelicity for proU in Escherichia coli.

Protein H1: a role for chromatin structure in the regulation of bacterial gene expression and virulence?

There has been a recent revival of interest in one of the most abundant Escherichia coli proteins, H1 (also called H-NS). This protein was first identified many years ago as a major component of the bacterial nucleoid, and has been characterized biochemically by several groups. However, no clear function for the protein emerged from these studies. Our thinking has been transformed by recent findings which complement the biochemistry with genetic data. Several mutations, selected over many years by virtue of their diverse effects on gene expression, have turned out to be allelic and to fall within the structural gene for H1. Bringing together the genetics and the biochemistry has demonstrated that the whole is worth more than the sum of the parts! These findings have far-reaching implications for the mechanisms by which gene expression is regulated and also, perhaps, for the control of bacterial virulence.

Transposon tagging of genes for cell-cell interactions in Myxococcus xanthus

The prokaryote Myxococcus xanthus is a model for cell interactions important in multicellular behavior. We used the transposon TnphoA to specifically identify genes for cell-surface factors involved in cell interactions. From a library of 10,700 insertions of TnphoA, we isolated 36 that produced alkaline phosphatase activity. Three TnphoA insertions tagged cell motility genes, called cgl, which control the adventurous movement of cells. The products of the tagged cgl genes could function in trans upon other cells and were localized primarily in the cell envelope and extracellular space, consistent with TnphoA tagging genes for extracellular factors controlling motility.

Analysis of protein expression in response to osmotic stress in Escherichia coli

Two strains of Escherichia coli isogenic except for the cya (adenylate cyclase) allele were grown with [35S]methionine and cysteine in minimal defined glucose medium and in this medium with 600 mM NaCl to induce osmotic stress. Cells were grown for approximately two generations. The labeled proteins were separated by 2-dimensional electrophoresis and were quantified fluorographically. Of the 263 major proteins (proteins incorporating 0.10% or more of the total radioactivity) in the cya+ control culture, radioactivity in 41 proteins was at least ten times greater in cells grown with osmotic stress. Six of these individual proteins each accounted for 1.0% or more of the total radioactive label in the cells. Conversely, radioactivity in 31 major proteins appeared to decrease at least ten times when cells grew with osmotic stress. These data indicate that the response of the bacterium to osmotic stress involves induction of some proteins and repression of others. 61% of the proteins that appear to be stimulated by salt stress were found in both strains indicating there is no obligatory requirement for cAMP.

Nucleotide sequence and analysis of the Vibrio alginolyticus sucrose uptake-encoding region

The nucleotide sequence of the Vibrio alginolyticus sucrose uptake-encoding region was determined, and contained two genes, scrA and scrK. The scrA gene encodes an enzyme IISucrose (EIIScr) protein of the phosphoenolpyruvate dependent phosphotransferase system and the scrK gene encodes a fructokinase. The deduced amino acid (aa) sequence for the V. alginolyticus EIIScr protein was homologous with the EIIScr proteins from Streptococcus mutans, Salmonella typhimurium (pUR400 system) and Bacillus subtilis. The deduced aa sequence for the V. alginolyticus fructokinase was homologous with the Escherichia coli enzymes, 6-phosphofructokinase (isoenzyme 2) and ribokinase. Transposon phoA mutagenesis experiments indicated that the EIIScr protein was a membrane-bound protein with a region that extended into the periplasm.

Genetic analysis of membrane protein topology by a sandwich gene fusion approach

We describe a cloning vector that allows the construction of phoA sandwich fusions in which mature alkaline phosphatase is inserted into target proteins. In contrast to previous fusions obtained using the TnphoA transposon, the entire amino acid sequence of the target protein is present in the fusion product. We have constructed a series of sandwich fusions of alkaline phosphatase to the multispanning cytoplasmic membrane protein MalF. Despite the fact that the alkaline phosphatase was tethered to MalF at both its N and its C terminus, the enzyme exhibited high activity when it was fused to a periplasmic domain of the membrane protein. Cells harboring an alkaline phosphatase sandwich fusion to the end of the first membrane-spanning segment of MalF exhibited both MalF and alkaline phosphatase activity. When alkaline phosphatase was inserted into a cytoplasmic domain of MalF, its specific activity was very low. Our results suggest that the alkaline phosphatase activity of phoA sandwich fusions provides a more sensitive monitor than previous methods of the cellular localization of the domain of the target protein to which the enzyme is fused. Thus, the sandwich fusion approach can give a more accurate picture of membrane protein topology.

The osmZ (bglY) gene encodes the DNA-binding protein H-NS (H1a), a component of the Escherichia coli K12 nucleoid

A class of trans-acting mutations, which alter the osmoregulated expression of the Escherichia coli proU operon, maps at 27 min on the chromosome in a locus we have called osmZ. Mutations in osmZ are allelic to bglY, pilG and virR, affect gene expression, increase the frequency of the site-specific DNA inversion mediating fimbrial phase variation, stimulate the formation of deletions, and influence in vivo supercoiling of reporter plasmids. We have cloned the osmZ+ gene, mapped it at 1307 kb of the E. coli restriction map, identified its gene product as a 16 kDa protein, and determined the nucleotide sequence of the osmZ+ gene. The deduced amino acid sequence for OsmZ predicts a protein of 137 amino acid residues with a calculated molecular weight of 15,530. The primary sequence of OsmZ is identical to that of H-NS (H1a), a DNA-binding protein that affects DNA topology and is known to be associated with the bacterial nucleoid. Thus, osmZ is the structural gene for the H-NS (H1a) protein. The nucleotide sequence of osmZ is almost identical to that of hns; however, hns was incorrectly located at 6.1 min on the E. coli linkage map. Increased osmZ gene dosage leads to cell filament formation, altered gene expression, and reduced frequency of fimbrial phase variation. Our results suggest that the nucleoid-associated DNA-binding protein H-NS (H1a) plays a critical role in gene expression and in determining the structure of the genetic material.

Bacillus subtilis holo-cytochrome c-550 can be synthesised in aerobic Escherichia coli

Bacillus subtilis membrane-bound holo-cytochrome c-550 was found to be expressed from the structural gene cloned on a plasmid vector in aerobically grown Escherichia coli and exhibited normal biochemical properties. This occurs despite the lack of endogenous cytochrome c and suggests that cytochrome c-heme lyase activity is also present in aerobic E. coli. The membrane topology of B. subtilis cytochrome c-550 was studied using fusions to alkaline phosphatase (PhoA). The results show that the heme domain (at least when fused to PhoA) can be translocated as apo-cytochrome and confirm that the N-terminal part of the cytochrome functions as both export signal and membrane anchor for the C-terminal heme domain. A model for the organisation of B. subtilis cytochrome c-550 in the cytoplasmic membrane is presented.

Sequencing and expression of a cellodextrinase (ced1) gene from Butyrivibrio fibrisolvens H17c cloned in Escherichia coli

The nucleotide sequence of a 2.314 kb DNA segment containing a gene (ced1) expressing cellodextrinase activity from Butyrivibrio fibrisolvens H17c was determined. The B. fibrisolvens H17c gene was expressed from a weak internal promoter in Escherichia coli and a putative consensus promoter sequence was identified upstream of a ribosome binding site and a GTG start codon. The complete amino acid sequence (547 residues) was deduced and homology was demonstrated with the Clostridium thermocellum endoglucanase D (EGD), Pseudomonas fluorescens var. cellulosa endoglucanase (EG), and a cellulase from the avocado fruit (Persea americana). The ced1 gene product Ced1 showed cellodextrinase activity and rapidly hydrolysed short-chain cellodextrins to yield either cellobiose or cellobiose and glucose as end products. The Ced1 enzyme released cellobiose from p-nitrophenyl-beta-D-cellobioside and the enzyme was not inhibited by methylcellulose, an inhibitor of endoglucanase activity. Although the major activity of the Ced1 enzyme was that of a cellodextrinase it also showed limited activity against endoglucanase specific substrates [carboxymethylcellulose (CMC), lichenan, laminarin and xylan]. Analysis by SDS-polyacrylamide gel electrophoresis with incorporated CMC showed a major activity band with an apparent Mr of approximately 61,000. The calculated Mr of the ced1 gene product was 61,023.

lac permease of Escherichia coli: topology and sequence elements promoting membrane insertion

The membrane topology of Escherichia coli lac permease was analyzed using a set of 36 lac permease-alkaline phosphatase (lacY-phoA) gene fusions. The level of enzymatic activity of alkaline phosphatase fused to a cytoplasmic membrane protein appears to reflect whether the fusion junction site normally faces the cytoplasm or periplasm. The alkaline phosphatase activities of cells expressing the lacY-phoA fusions distinguish between models previously proposed for the topology of lac permease and favor one with 12 transmembrane segments. This model is fully compatible with the results of earlier biochemical and immunological studies. The properties of fusions with junctions spanning two of the transmembrane segments at 2- or 3-amino acid intervals indicate that approximately half of the residues of either segment (9-11 amino acids) suffices to promote alkaline phosphatase translocation across the membrane. The additional transmembrane segment amino acids that are not required for this membrane insertion process may normally be needed in unfused lac permease after insertion for stable association with the membrane.

Nucleotide sequence of the celA gene encoding a cellodextrinase of Ruminococcus flavefaciens FD-1

The nucleotide sequence of a 3.6 kb DNA fragment containing a cellodextrinase gene (celA) from Ruminococcus flavefaciens FD-1 was determined. The gene was expressed from its own regulatory region in Escherichia coli and a putative consensus promoter sequence was identified upstream of a ribosome binding site and a TTG start codon. The complete amino acid sequence of the CelA enzyme (352 residues) was deduced and showed no significant homology to cellulases from other organisms. Two lysozyme-type active sites were found in the amino-terminal third of the enzyme. In E. coli the cloned Cel A protein was translocated into the periplasm. The lack of a typical signal sequence, and the results of transposon phoA mutagenesis experiments indicated that CelA is secreted by a mechanism other than a leader peptide.

Enteric bacteria and osmotic stress: intracellular potassium glutamate as a secondary signal of osmotic stress?

Enteric bacteria have evolved an impressive array of mechanisms that allow the cell to grow at widely different external osmotic pressures. These serve two linked functions; firstly, they allow the cell to maintain a relatively constant turgor pressure which is essential for cell growth; and secondly they permit changes in cytoplasmic composition such that the accumulation of intracellular osmolytes required to restore turgor pressure does not impair enzyme function. The primary event in turgor regulation is the controlled accumulation of potassium and its counterion glutamate. At high external osmolarities the cytoplasmic levels of potassium glutamate can impair enzyme function. Rapid growth is therefore dependent upon secondary responses, principally the accumulation of compatible solutes, betaine (N-trimethylglycine), proline and trehalose. The accumulation of these solutes is achieved by the controlled activity of transport systems and enzymes in response to changes in external osmotic pressure. It has been proposed that the accumulation of potassium glutamate during turgor regulation acts as a signal for the activation of these systems [1,2]. This brief review will examine the evidence that control over the balance of cytoplasmic osmolytes is achieved by sensing of the intracellular potassium (and glutamate) concentration.

Regulation of divergent transcription from the iron-responsive fepB-entC promoter-operator regions in Escherichia coli

Transcriptional linkage of the enterobactin gene cluster entCEBA (P15) was confirmed by ent-lacZ gene fusion analysis. Control sequences directing iron-regulated expression of this polycistronic message were localized to the fepB-entC bidirectional promoter region. Transcriptional initiation sites defined by primer extension analysis were located 103 base-pairs apart for the divergent fepB and entC messages. Within this divergent regulatory region, strongly consensus -35 and -10 promoter determinants and potential Fur repressor-binding sequences were identified. A vector containing divergently oriented indicator gene fusions was constructed to monitor regulatory effects of mutations within this iron-responsive control region. The fepB-entC promoter-operator elements were confirmed by mutation, using the dual gene fusion system in multicopy and low copy number states. Mutations in the -35 and -10 regions of the fepB and entC promoters that decreased their similarity to consensus resulted in reduced promoter activity. Mutations in the Fur-controlled operators reduced induction ratios (iron-deficient levels/iron-rich levels) for the respective fusion gene activities by approximately sevenfold. Although operator mutants retained some degree of inducibility, complete relief of repression was observed for double operator mutants, suggesting that only minor regulatory influence is exerted by Fur occupation of the opposing operator site. DNase I footprinting experiments were performed to characterize the sequence-specific Fur interactions at the operator sequences. At the fepB operator, a 31 base-pair Fur-protected region was identified, corresponding to positions -19 to +12 with respect to the transcriptional start site. Similarly, Fur protected a 31 base-pair region in entC, corresponding to positions +1 to +31 in the message. A contiguous and sequentially occupied secondary Fur-binding site in entC was protected at higher Fur concentrations, extending the protected region to +49, and sequestering the putative Shine-Dalgarno sequence. Operator positional effects and co-operativity are discussed.

Alkaline phosphatase fusions: sensors of subcellular location

Alkaline phosphatase fusions allow genes to be identified solely on the basis of their protein products being exported from the cytoplasm. Thus, the use of such fusions helps render biological processes which involve cell envelope and secreted proteins accessible to a sophisticated genetic analysis. Furthermore, alkaline phosphatase fusions can be used to locate export signals. Specifying such signals is an important component of studies on the structure of individual cell envelope proteins. The basis of the alkaline phosphatase fusion approach is the finding that the activity of the enzyme responds differently to different environments. Thus, the activity of the fusion protein gives evidence as to its location. This general approach of using sensor proteins which vary in their function, depending on their environment, could be extended to the study of other sorts of problems. It may be that certain enzymes will provide an assay for localization to a particular subcellular compartment, if the environment of the compartment differs from that of others. For instance, the lysosome is more acidic than other intracellular organelles. A gene fusion system employing a reporter enzyme that could show activity only at the pH of the lysosome could allow the detection of signals determining lysosomal localization. Analogous types of enzymes may be used as probes for other subcellular compartments.

mprA, an Escherichia coli gene that reduces growth-phase-dependent synthesis of microcins B17 and C7 and blocks osmoinduction of proU when cloned on a high-copy-number plasmid

Microcins B17 and C7 are plasmid-determined, peptide antibiotics produced by Escherichia coli when cells enter the stationary phase of growth. Microcinogenic strains are immune to the action of the microcin they synthesize. A well-characterized deficient-immunity phenotype is exhibited by microcin B17-producing cells in the absence of the immunity gene mcbG (M.C. Garrido, M. Herrero, R. Kolter, and F. Moreno, EMBO J. 7:1853-1862, 1988). A 14.6-kilobase-pair EcoRI chromosomal fragment was isolated by its ability to suppress this phenotype when cloned into a multicopy vector. This fragment was mapped to 57.5 min on the E. coli genetic map. The position of the gene responsible for suppression, designated mprA, was determined by insertional mutagenesis and deletion analysis. mprA was shown to be transcribed clockwise on the E. coli chromosome, and its product was identified as a 19-kilodalton polypeptide. Suppression was shown to be achieved by decreasing microcin B17 production. Increased mprA gene dosage also caused a decrease in microcin C7 production and blocked the osmoinduction of the proU locus in high-osmolarity media. Our results suggest that the mprA gene product could play a regulatory role on expression of several E. coli genes, this control being exerted at the transcriptional level.

The 987P fimbrial gene cluster of enterotoxigenic Escherichia coli is plasmid encoded

A clone containing the 987P fimbrial gene cluster was selected from a cosmid library of total DNA of the prototype Escherichia coli strain 987 by using 987P-specific antiserum. A subclone of 12 kilobases containing all of the genes required for fimbrial expression on a nonfimbriated K-12 strain of E. coli and a DNA fragment internal to the fimbrial subunit gene were used to probe the prototype strain and various isolates of 987P-fimbriated enterotoxigenic E. coli. All strains had several plasmids, as shown by agarose gel electrophoresis, and each of five strains which expressed 987P fimbriae showed a plasmid of 35 to 40 megadaltons (MDa) hybridizing to both 987P-specific probes. Hybridization to restricted DNA of strain 987 supported a plasmid origin for the cloned 987P gene cluster. Moreover, an isogenic strain which had lost its 35-MDa plasmid was no longer capable of synthesizing fimbrial subunits, but regained fimbrial expression after reintroduction of the TnphoA (Tn5 IS50L::phoA)-tagged 35-MDa plasmid. Absence of fimbrial subunit synthesis in K-12 strains transformed with the 35-MDa plasmid alone suggested the requirement of regulatory elements existing in strain 987 but missing in K-12 strains. A probe for the heat-stable enterotoxin STIa hybridized in each of the 987P-fimbriated strains to the plasmid containing the 987P genes and in most of these strains to an additional plasmid which contained the gene for the heat-stable enterotoxin STII. Occurrence of the 987P and STIa genes on the same replicon correlates with epidemiological observations, STIa being the most prevalent toxin produced by 987P-fimbriated E. coli.

Glycerol facilitator of Escherichia coli: cloning of glpF and identification of the glpF product

The glycerol facilitator is known as the only example of a transport protein that catalyzes facilitated diffusion across the Escherichia coli inner membrane. Here we show that the gene encoding the facilitator, glpF, is the first gene in an operon with glpK, encoding glycerol kinase, at 88 min of the E. coli chromosome. The operon is transcribed counterclockwise. We cloned the glpF gene, demonstrated that it complemented a chromosomal glycerol transport-minus mutation, and identified the gene product. The GlpF protein appeared in the membrane fraction of plasmid-bearing strains and had an apparent Mr of 25,000.

Characterization of the osmoregulated Escherichia coli proU promoter and identification of ProV as a membrane-associated protein

The Escherichia coli proU operon encodes a high-affinity, binding-protein-dependent transport system for the osmoprotectant glycine betaine. Expression of proU is osmoregulated, and transcription of this operon is greatly increased in cells grown at high osmolarity. Characterization of the proU operon and its promoter provided results similar to those published elsewhere (Gowrishankar, 1989; Stirling et al., 1989). The previously identified proU601 mutation, which leads to increased proU expression both at low- and high osmolarity, is a G to A transition in the Pribnow box of the proU promoter, which increases the homology of the -10 region to the consensus sequence of E. coli promoters. Using an antiserum raised against a ProV-beta-galactosidase hybrid protein, we have identified ProV as a protein associated with the cytoplasmic membrane. This cellular location is consistent with its proposed role as the energy-coupling component of the ProU transport system.

Export of FepA::PhoA fusion proteins to the outer membrane of Escherichia coli K-12

A library of fepA::phoA gene fusions was generated in order to study the structure and secretion of the Escherichia coli K-12 ferric enterobactin receptor, FepA. All of the fusion proteins contained various lengths of the amino-terminal portion of FepA fused in frame to the catalytic portion of bacterial alkaline phosphatase. Localization of FepA::PhoA fusion proteins in the cell envelope was dependent on the number of residues of mature FepA present at the amino terminus. Hybrids containing up to one-third of the amino-terminal portion of FepA fractionated with their periplasm, while those containing longer sequences of mature FepA were exported to the outer membrane. Outer membrane-localized fusion proteins expressed FepA sequences on the external face of the outer membrane and alkaline phosphatase moieties in the periplasmic space. From sequence determinations of the fepA::phoA fusion joints, residues within FepA which may be exposed on the periplasmic side of the outer membrane were identified.

Use of phoA fusions to study the topology of the Escherichia coli inner membrane protein leader peptidase

A topology of the Escherichia coli leader peptidase has been previously proposed on the basis of proteolytic studies. Here, a collection of alkaline phosphatase fusions to leader peptidase is described. Fusions to the periplasmic domain of this protein exhibit high alkaline phosphatase activity, while fusions to the cytoplasmic domain exhibit low activity. Elements within the cytoplasmic domain are necessary to stably anchor alkaline phosphatase in the cytoplasm. The amino-terminal hydrophobic segment of leader peptidase acts as a weak export signal for alkaline phosphatase. However, when this segment is preceded by four lysines, it acts as a highly efficient export signal. The coherence of in vitro studies with alkaline phosphatase fusion analysis of the topology of leader peptidase further indicates the utility of this genetic approach to membrane protein structure and insertion.

Genetic analysis of bacteriophage N4 adsorption

We isolated six mutants of Escherichia coli K-12 that were defective in bacteriophage N4 adsorption. We mapped the mutations to four loci designated nfrA through nfrD (N four resistance). nfrA and nfrB were tightly linked to each other and were mapped to min 12 of the E. coli linkage map. nfrC was mapped to min 85, and nfrD was mapped between min 44 and 58. We isolated a clone carrying both nfrA and nfrB and identified its gene products through maxicell analysis of plasmid subclones. The nfrA gene product was an outer membrane protein of 96,000 apparent molecular weight, whereas nfrB encoded an inner-membrane protein of 69,500 apparent molecular weight. The nfrB1 mutation did not affect the export of the nfrA gene product to the outer membrane and did not affect the alkaline phosphatase activity of an nfrA-phoA fusion. We propose that nfrA encodes the structural receptor for N4 and that the nfrB gene product may be required for irreversible adsorption and injection of the phage genome and virion-encapsulated RNA polymerase through the inner membrane.

MalI, a novel protein involved in regulation of the maltose system of Escherichia coli, is highly homologous to the repressor proteins GalR, CytR, and LacI

The maltose regulon of Escherichia coli comprises several operons that are under common regulatory control of the MalT activator protein. Five mal genes, organized in two divergent operons, code for a binding-protein-dependent transport system specific for maltose and maltodextrins. MalK, one of the subunits of this transport system, not only is essential for transport but also plays a role in regulation. Mutations abolishing MalK function not only result in inability to transport maltose but also cause constitutive expression of the maltose regulon. For this constitutivity to be exerted, the function of an additional gene product, MalI, is necessary. Using the constitutive expression of a malK-lacZ fusion as a signal, we cloned the malI gene, expressed it in minicells, and determined its DNA sequence. The sequence predicted a protein of 34,729 molecular weight, in agreement with the apparent molecular weight of the protein (35,000) when expressed in minicells and analyzed by polyacrylamide gel electrophoresis and autoradiography. MalI exhibited high homology to the repressor proteins GalR, CytR, and LacI. When the amino acid sequences were appropriately aligned, MalI showed 28% identity to GalR, 21% to CytR, and 24% to LacI. Including conservative amino acid exchanges, these numbers increased to 69, 56, and 58%, respectively. The regions of high homology were clustered in particular at the N-terminal portion of the protein that includes the helix-turn-helix motif thought to be involved in DNA binding. The protein contained a short stretch of 30 amino acids that was surprisingly homologous to a sequence in MalT. The amino-terminal half of the protein exhibited significant homology with MalK. The transcriptional start of malI was determined by reverse transcriptase and by S1 nuclease mapping. We found a possible binding site for cyclic AMP receptor protein in the promoter region of malI as well as two perfect direct repeats of 14 base pairs with twofold symmetry indicating their possible role as operator sites. Upstream to malI we observed a divergent open reading frame that extended to the end of the sequenced DNA.

Genetic studies on the inability of beta-galactosidase to be translocated across the Escherichia coli cytoplasmic membrane

When a signal sequence is attached to beta-galactosidase, the normally cytoplasmic protein is unable to fully traverse the cytoplasmic membrane. We used a genetic approach to study those features of beta-galactosidase responsible for the block in translocation. By using both in vivo and in vitro techniques, fragments of beta-galactosidase were interposed between a signal sequence and alkaline phosphatase. The alkaline phosphatase acts as a sensor for any blocking effects of beta-galactosidase on export. From these studies, we show that multiple regions of beta-galactosidase contribute to its failure to be translocated. These results are most easily interpreted if the folding of beta-galactosidase or of domains of it is responsible for the block in export. In addition, in certain constructs, positively charged amino acids directly following the signal sequence interfered with export.

Cloning and molecular characterization of genes whose products allow Salmonella typhimurium to penetrate tissue culture cells

Invasion of the intestinal epithelium is thought to be an important step in the pathogenesis of Salmonella infections. Using an in vitro system, we have isolated a genetic locus, inv, that confers to a noninvasive strain of Salmonella typhimurium the ability to penetrate tissue culture cells. Highly virulent S. typhimurium strains carrying inv mutations were defective for entry into Henle-407 cells while remaining unaffected in their ability to attach to cultured cells. When administered perorally to BALB/c mice, inv mutants of S. typhimurium had higher 50% lethal doses (LD50) than their wild-type parent strains. To the contrary, there were no differences in the observed LD50 when strains were administered intraperitoneally. In addition, inv mutants presented decreased ability to colonize the Peyer's patches, the small intestinal wall, and the spleen when administered perorally, although when administered intraperitoneally, they showed no difference in their ability to colonize the spleen compared to the wild-type parent strain.

Nucleotide sequence and analysis of the Vibrio alginolyticus sucrase gene (scrB)

The nucleotide sequence of a 2.119-kb DNA fragment containing the Vibrio alginolyticus sucrase gene (scrB) was determined. The complete sequence (484 aa residues) of the sucrase was deduced and homology was detected between the sucrase enzymes from V. alginolyticus and the Gram-positive bacteria Bacillus subtilis and Streptococcus mutans. In Escherichia coli cells the cloned V. alginolyticus sucrase is translocated to the periplasm. Transposon phoA mutagenesis experiments strongly suggested that V. alginolyticus sucrase in E. coli is not exported across the cytoplasmic membrane by means of a typical signal sequence.

Molecular characterization of the proU loci of Salmonella typhimurium and Escherichia coli encoding osmoregulated glycine betaine transport systems

The proU loci of Salmonella typhimurium and Escherichia coli encode high-affinity glycine betaine transport systems which play an important role in survival under osmotic stress. Transcription of the proU locus is tightly regulated by osmolarity and this regulation appears to be mediated by osmotically induced changes in DNA supercoiling. In order to study the regulatory mechanisms involved we have cloned and characterized the proU locus of S. typhimurium by an in vivo transductional procedure. The locus is shown to consist of at least three genes, designated proVWX, cotranscribed as a single operon. The first gene in the operon encodes a protein sharing considerable sequence identity with ATP-binding proteins from other periplasmic transport systems. Unexpectedly, the highly expressed periplasmic glycine betaine binding protein was found to be encoded by a distal gene, proX, in the operon. The operon has no significant internal promoters but is expressed from a single osmoregulated promoter whose transcription start site has been mapped. The proU promoter of E. coli has also been sequenced and the transcription start site shown to be similar to that of S. typhimurium. Evidence is presented which suggests that, besides de novo glycine betaine uptake, an important function of ProU may be the recapture and recycling of other osmolytes that leak from the cell.