Isolation and sequencing of Escherichia coli gene proP reveals unusual structural features of the osmoregulatory proline/betaine transporter, ProP

Cloning and characterization of a Campylobacter jejuni 72Dz/92 gene encoding a 30 kDa immunopositive protein, component of the ABC transport system; expression of the gene in avirulent Salmonella typhimurium

Three gene libraries of Campylobacter jejuni 72Dz/92 DNA were prepared using lambda gt11, pSupercos and pWSK129 cloning vectors. Screening of the libraries with Escherichia coli absorbed antiserum generated against whole C. jejuni revealed several immunoreactive clones of apparent molecular masses 19, 28, 30 and 50 kDa. The most commonly isolated clones expressed 30 kDa protein. The nucleotide sequence of the 1768 bp C. jejuni DNA yielded one complete (ORF2) and two partial open reading frames (ORF1 and ORF3). ORF2 encoded CjaA protein exhibits relevant overall homology to several prokaryotic solute binding proteins (family 3), components of the ABC transport system, while the product of the truncated ORF3 (CjaB protein) shows extensive homology to Gram-negative bacterial proteins, members of the sugar transporter family. The genetic organization of the putative cjaAB operon was studied. The cjaA gene fragment (616 bp) was amplified from three C. jejuni strains isolated from patients with acute bloody diarrhea, whereas it was not amplified from strains which caused acute diarrhea with no blood in the stools. The gene was introduced into avirulent Salmonella typhimurium vaccine strain where it is expressed at a reasonably high level.

Osmotically induced response in representatives of halophilic prokaryotes: the bacterium Halomonas elongata and the archaeon Haloferax volcanii

Haloferax volcanii and Halomonas elongata have been selected as representatives of halophilic Archaea and Bacteria, respectively, to analyze the responses to various osmolarities at the protein synthesis level. We have identified a set of high-salt-related proteins (39, 24, 20, and 15.5 kDa in H. elongata; 70, 68, 48, and 16 kDa in H. volcanii) whose synthesis rates increased with increasing salinities. A different set of proteins (60, 42, 15, and 6 kDa for H. elongata; 63, 44, 34, 18, 17, and 6 kDa for H. volcanii), some unique for low salinities, was induced under low-salt conditions. For both organisms, and especially for the haloarchaeon, adaptation to low-salt conditions involved a stronger and more specific response than adaptation to high-salt conditions, indicating that unique mechanisms may have evolved for low-salinity adaptation. In the case of H. volcanii, proteins with a typical transient response to osmotic shock, induced by both hypo- and hyperosmotic conditions, probably corresponding to described heat shock proteins and showing the characteristics of general stress proteins, have also been identified. Cell recovery after a shift to low salinities was immediate in both organisms. In contrast, adaptation to higher salinities in both cases involved a lag period during which growth and general protein synthesis were halted, although the high-salt-related proteins were induced rapidly. In H. volcanii, this lag period corresponded exactly to the time needed for cells to accumulate adequate intracellular potassium concentrations, while extrusion of potassium after the down-shift was immediate. Thus, reaching osmotic balance must be the main limiting factor for recovery of cell functions after the variation in salinity.

Activation of RpoS-dependent proP P2 transcription by the Fis protein in vitro

The proP gene, encoding a transporter of the osmoprotecting compounds proline and glycine betaine, is expressed from two promoters. Transcription of the P2 promoter occurs at a transient period in late exponential phase and is dependent upon Fis and the RpoS (sigma38) sigma factor. Here we characterize Fis-mediated activation of the P2 promoter in vitro. We find that this promoter displays unusually high specificity for sigma38. Fis strongly activates P2 when bound to site I centered at -41 within the promoter region. There is a complex relationship involving DNA supercoiling and potassium glutamate concentration on Fis activation, but most efficient transcription occurs under high salt conditions when the superhelical density is above -0.03. The major stimulatory effect of DNA supercoiling occurs between superhelical densities of 0 to -0.02 suggesting that, while supercoiling is mechanistically important, it may not be a physiologically relevant controlling factor. However, the stimulation of transcription by high potassium glutamate concentrations may contribute to the osmotic inducibility of the P2 promoter. We show that Fis and E sigma38 bind cooperatively on supercoiled DNA to form a stable complex at P2 that involves promoter melting. Fis also binds to a second site within the proP regulatory region. While binding to this site appears to play no role in Fis activation of the P2 promoter, it functions as a repressor of transcription initiating from the P1 promoter by either sigma70 or sigma38.

"Compensatory" organic osmolytes in high osmolarity and dehydration stresses: history and perspectives

As stated in the conclusion, "life is a thing of macromolecular cohesion in salty water." This brief historical overview shows that "compensatory" organic osmolytes take an essential place in this cohesion. It reviews the major steps of the study of these compounds over more than 100 years, from the early beginnings of 1885 until now, showing some of its fascinating developments and ending on the idea that the most fascinating is still to come. This study can be taken as an example of the richness of the comparative approach.

Cyclic AMP receptor protein functions as a repressor of the osmotically inducible promoter proP P1 in Escherichia coli

Transcription of the proP gene, encoding a transporter of the osmoprotectants proline and glycine betaine, is controlled from two promoters, P1 and P2, that respond primarily to osmotic and stationary-phase signals, respectively. The P1 promoter is normally expressed at a very low level under low or normal medium osmolarity. We demonstrate that the binding of the cyclic AMP (cAMP) receptor protein (CRP) to a site centered at -34.5 within the promoter is responsible for the low promoter activity under these conditions. A brief period of reduced CRP binding in early log phase corresponds to a transient burst of P1 transcription upon resumption of growth in Luria-Bertani broth. A CRP binding-site mutation or the absence of a functional crp gene leads to high constitutive expression of P1. We show that the binding of CRP-cAMP inhibits transcription by purified RNA polymerase in vitro at P1, but this repression is relieved at moderately high potassium glutamate concentrations. Likewise, open-complex formation at P1 in vivo is inhibited by the presence of CRP under low-osmolarity conditions. Because P1 expression can be further induced by osmotic upshifts in a delta crp strain or in the presence of the CRP binding-site mutation, additional controls exist to osmotically regulate P1 expression.

Use of phoA and lacZ fusions to study the membrane topology of ProW, a component of the osmoregulated ProU transport system of Escherichia coli

The Escherichia coli ProU system is a member of the ATP-binding cassette (ABC) superfamily of transporters. ProU consists of three components (ProV, ProW, and ProX) and functions as a high-affinity, binding protein-dependent transport system for the osmoprotectants glycine betaine and proline betaine. The ProW protein is the integral inner membrane component of the ProU system. Its hydropathy profile predicts seven transmembrane spans and a hydrophilic amino terminus of approximately 100 residues, and it suggests the presence of an amphiphilic alpha-helix (L-61 to F-97) in close proximity to the first strongly hydrophobic segment of ProW. We have studied the membrane topology of the ProW protein by the phoA and lacZ gene fusion approach. A collection of 10 different proW-phoA fusions with alkaline phosphatase activity and 8 different proW-lacZ fusions with beta-galactosidase activity were isolated in vivo after TnphoAB and TnlacZ mutagenesis of a plasmid-encoded proW gene. The recovery of both enzymatically active ProW-PhoA and ProW-LacZ hybrid proteins indicates that segments of ProW are exposed on both sides of the cytoplasmic membrane. To compare the enzymatic activities of each of the indicator proteins joined at a particular site in ProW, we switched the phoA and lacZ reporter genes in vitro in each of the originally in vivo-isolated gene fusions. A mirror-like pattern in the enzyme activity of the resulting new ProW-PhoA and ProW-LacZ hybrid proteins emerged, thus providing positive signals for the location of both periplasmic and cytoplasmic domains in ProW. The protease kallikrein digests the amino-terminal tail of a ProW-LacZ hybrid protein in spheroplasts, suggesting that the amino terminus of ProW is located on the periplasmic side of the cytoplasmic membrane. From these data, a two-dimensional model for ProW was constructed; this model consists of seven transmembrane alpha-helices and an unusual amino-terminal tail of approximately 100 amino acid residues that protrudes into the periplasmic space.

Isolation, characterization, and expression of the Corynebacterium glutamicum betP gene, encoding the transport system for the compatible solute glycine betaine

Corynebacterium glutamicum accumulates glycine betaine under conditions of high osmolarity. Previous work revealed the existence of a high-affinity glycine betaine permease which is osmotically regulated. In the present study, the corresponding gene was cloned. The betP gene, encoding the glycine betaine uptake carrier, was isolated by heterologous complementation of mutant strain Escherichia coli MKH13. From sequence analysis it is predicted to encode a protein of 595 amino acids. This protein shares 36% identity with the choline transport system BetT and 28% identity with the carnitine transport system CaiT of E. coli, as well as 38% identity with a protein with an unknown function from Haemophilus influenzae. Analysis of hydropathy indicated a common structure for all four transport proteins. After heterologous expression of betP in E. coli MKH13, the measured Km values for glycine betaine and the cotransported Na+ were similar to those found in C. glutamicum, whereas the modulation of activity by osmotic gradients was shifted to lower osmotic values.

Sequences in the -35 region of Escherichia coli rpoS-dependent genes promote transcription by E sigma S

sigma S is an alternate sigma factor which functions with RNA polymerase to activate transcription of genes that are involved in a number of stress responses, including stationary-phase survival and osmoprotection. The similarity of the sigma S protein to sigma D (Escherichia coli's major sigma factor) in the regions thought to recognize and bind promoter sequences suggests that sigma S- and sigma D-associated RNA polymerases recognize promoter DNA in a similar manner. However, no promoter recognition sequence for sigma S holoenzyme (E sigma S) has been identified. An apparent conservation of cytosine nucleotides was noted in the -35 region of several sigma S-dependent promoters. Site-directed mutagenesis and reporter gene fusions were used to investigate the importance of the -35 cytosine nucleotides for sigma S-dependent transcription. Substitution of cytosine nucleotides for thymidine at the -35 site of the sigma D-dependent proU promoter effectively abolished transcription by E sigma D but allowed E sigma S to direct transcription from the mutant promoter. Inclusion of the sigma D consensus -10 hexamer strengthened transcription by E sigma S, demonstrating that both E sigma D and E sigma S can recognize the same -10 sequences. Conversely, replacement of -35 site cytosine nucleotides with thymidine in the sigma S-dependent osmY promoter reduced transcription by E sigma S and increased transcription by E sigma D. Our data suggest that DNA sequences in the -35 region function as part of a discriminator mechanism to shift transcription between E sigma D and E sigma S.

Characterization of the Erwinia chrysanthemi osmoprotectant transporter gene ousA

Growth of Erwinia chrysanthemi in media of elevated osmolarity can be achieved by the uptake and accumulation of various osmoprotectants. This study deals with the cloning and sequencing of the ousA gene-encoded osmoprotectant uptake system A from E. chrysanthemi 3937. OusA belongs to the superfamily of solute ion cotransporters. This osmotically inducible system allows the uptake of glycine betaine, proline, ectoine, and pipecolic acid and presents strong similarities in nucleotide sequence and protein function with the proline/betaine porter of Escherichia coli encoded by proP. The control of ousA expression is clearly different from that of proP. It is induced by osmotic strength and repressed by osmoprotectants. Its expression in E. coli is controlled by H-NS and is rpoS dependent in the exponential phase but unaffected by the stationary phase.

An overview of membrane transport proteins in Saccharomyces cerevisiae

All eukaryotic cells contain a wide variety of proteins embedded in the plasma and internal membranes, which ensure transmembrane solute transport. It is now established that a large proportion of these transport proteins can be grouped into families apparently conserved throughout organisms. This article presents the data of an in silicio analysis aimed at establishing a preliminary classification of membrane transport proteins in Saccharomyces cerevisiae. This analysis was conducted at a time when about 65% of all yeast genes were available in public databases. In addition to approximately 60 transport proteins whose function was at least partially known, approximately 100 deduced protein sequences of unknown function display significant sequence similarity to membrane transport proteins characterized in yeast and/or other organisms. While some protein families have been well characterized by classical genetic experimental approaches, others have largely if not totally escaped characterization. The proteins revealed by this in silicio analysis also include a putative K+ channel, proteins similar to aquaporins of plant and animal origin, proteins similar to Na+-solute symporters, a protein very similar to electroneural cation-chloride cotransporters, and a putative Na+-H+ antiporter. A new research area is anticipated: the functional analysis of many transport proteins whose existence was revealed by genome sequencing.

Mutational analysis and molecular modelling of an amino acid sequence motif conserved in antiporters but not symporters in a transporter superfamily

Elements of a 'G X8 G X3 G P X2 G G' amino acid sequence motif were conserved in the fifth predicted membrane-spanning domains of 31 antiporters, but none of 27 symporters or uniporters that together comprise a 'superfamily' of structurally, related transport proteins. Molecular modelling and mechanics predicted that the GP dipeptide of this motif bends the antiporters' fifth transmembrane helices, and that the repeating pattern of glycine residues forms a pocket, devoid of side chains, on the surface of these helices. The glycine residue in the motif's GP dipeptide was conserved in 90% of these antiporters with alanine being the only observed substitution. Replacement of the glycine residue of the GP dipeptide with alanine and serine reduced the level of tetracycline resistance conferred by TetA(C), a tetracycline/H+ antiporter, by 74 and 81%, respectively. All other substitutions totally abolished resistance to tetracycline. In contrast, replacement of the glycine residue of the GP dipeptide did not abolish increased susceptibility to cadmium, another phenotype conferred by TetA(C) independent of resistance to tetracycline. These results suggest that the glycine of the GP dipeptide is necessary for the tetracycline/H+ antiport activity of TetA(C), rather than its expression, stability, or general three-dimensional structure.

Fis activates the RpoS-dependent stationary-phase expression of proP in Escherichia coli

Fis is a general nucleoid-associated protein in Escherichia coli whose expression is highly regulated with respect to growth conditions. A random collection of transposon-induced lac fusions was screened for those which give increased expression in the presence of Fis in order to isolate a ProP-LacZ protein fusion. We find that proP, which encodes a low-affinity transporter of the important osmoprotectants proline and glycine betaine, is transcribed from two promoters. proP1 is transiently induced upon subculture and is upregulated by increases in medium osmolarity. As cells enter stationary phase, a second promoter, proP2, is strongly induced. This promoter can also be induced by high medium osmolarity in exponential phase. The activity of proP2 depends on Fis and the stationary-phase sigma factor sigmas. In the presence of Fis, proP2 expression is increased over 50-fold, as judged by the LacZ activity of cells carrying the proP-lacZ fusion as well as by direct RNA analysis, making this the most strongly activated promoter by Fis that has been described. Two Fis binding sites centered at positions -41 (site I) and -81 (site II) with respect to the transcription initiation site of P2 have been defined by DNase I footprinting. Mutations in site I largely abolish stationary-phase activation, while mutations at site II have a minor effect, suggesting that direct binding of Fis to site I is important for Fis-mediated activation of this promoter. In addition to Fis and sigmas, sequences located over 108 bp upstream of the proP2 transcription initiation site are required for efficient expression.

OpuA, an osmotically regulated binding protein-dependent transport system for the osmoprotectant glycine betaine in Bacillus subtilis

Exogenously provided glycine betaine can efficiently protect Bacillus subtilis from the detrimental effects of high osmolarity environments. Through functional complementation of an Escherichia coli mutant deficient in glycine betaine uptake with a gene library from B. subtilis, we have identified a multicomponent glycine betaine transport system, OpuA. Uptake of radiolabeled glycine betaine in B. subtilis was found to be osmotically stimulated and was strongly decreased in a mutant strain lacking the OpuA transport system. DNA sequence analysis revealed that the components of the OpuA system are encoded by anoperon (opuA) comprising three structural genes: opuAA, opuAB, and opuAC. The products of these genes exhibit features characteristic for binding protein-dependent transport systems and in particular show homology to the glycine betaine uptake system ProU from E. coli. Expression of the opuA operon is under osmotic control. The transcriptional initiation sites of opuA were mapped by high resolution primer extension analysis, and two opuA mRNAs were detected that differed by 38 base pairs at their 5' ends. Synthesis of the shorter transcript was strongly increased in cells grown at high osmolarity, whereas the amount of the longer transcript did not vary in response to medium osmolarity. Physical and genetic mapping experiments allowed the positioning the opuA operon at 25 degrees on the genetic map of B. subtilis.

The osmoprotectant proline betaine is a major substrate for the binding-protein-dependent transport system ProU of Escherichia coli K-12

The ProP and ProU transport systems of Escherichia coli mediate the uptake of several osmoprotectants including glycine betaine. Here we report that both ProP and ProU are involved in the transport of the potent osmoprotectant proline betaine. A set of isogenic E. coli strains carrying deletions in either the proP or proU loci was constructed. The growth properties of these mutants in high osmolarity minimal media containing 1 mM proline betaine demonstrated that the osmoprotective effect of this compound was dependent on either an intact ProP or ProU uptake system. Proline betaine competes with glycine betaine for binding to the proU-encoded periplasmic substrate binding protein (ProX) and we estimate a KD of 5.2 microM for proline betaine binding. This value is similar to the binding constant of the ProX protein determined previously for the binding of glycine betaine (KD of 1.4 microM). Our results thus demonstrate that the binding-protein-dependent ProU transport system of E. coli mediates the efficient uptake of the osmoprotectants glycine betaine and proline betaine.

Identification of a PutP proline permease gene homolog from Staphylococcus aureus by expression cloning of the high-affinity proline transport system in Escherichia coli

The important food-borne pathogen Staphylococcus aureus is distinguished by its ability to grow at low water activity values. Previous work in our laboratory and by others has revealed that proline accumulation via transport is an important osmoregulatory strategy employed by this bacterium. Furthermore, proline uptake by this bacterium has been shown to be mediated by two distinct transport systems: a high-affinity system and a low-affinity system (J.-H. Bae, and K. J. Miller, Appl. Environ. Microbiol. 58:471-475, 1992; D. E. Townsend and B. J. Wilkinson, J. Bacteriol. 174:2702-2710, 1992). In the present study, we report the cloning of the high-affinity proline transport system of S. aureus by functional expression in an Escherichia coli host. The sequence of the staphylococcal proline permease gene was predicted to encode a protein of 497 amino acids which shares 49% identity with the PutP high-affinity proline permease of E. coli. Analysis of hydropathy also indicated a common overall structure for these proteins.

Two different Escherichia coli proP promoters respond to osmotic and growth phase signals

proP of Escherichia coli encodes an active transport system for proline and glycine betaine which is activated by both hyperosmolarity and amino acid-limited growth. proP DNA sequences far upstream from the translational start site are strongly homologous to the promoter of proU, an operon that specifies another osmoregulated glycine betaine transport system. Mutation and deletion analysis of proP and primer extension experiments established that this promoter, P1, was responsible for proP's strong expression in minimal medium and its response to osmotic signals. When cells were grown in complex medium, expression from a proP-lacZ fusion was induced three- to fourfold as growth slowed and cells entered stationary phase. Stationary-phase induction was dependent on rpoS, which encodes a stationary-phase sigma factor. Deletion of 158 bp of the untranslated leader sequence between P1 and the proP structural gene abolished rpoS-dependent stationary-phase regulation. Transcription initiation detected by primer extension within this region was absent in an rpoS mutant. proP is therefore a member of the growing class of sigma S-dependent genes which respond to both stationary-phase and hyperosmolarity signals.

pOSEX: vectors for osmotically controlled and finely tuned gene expression in Escherichia coli

Expression of the proU operon of Escherichia coli is directly proportional to the osmolarity of the growth medium. The basal level of proU transcription is very low, but a large increase is triggered by a sudden rise in the external osmolarity. This increased expression is maintained for as long as the osmotic stimulus persists. We have capitalized upon these regulatory features of the proU operon and have constructed a series of expression vectors (pOSEX) permitting osmotically controlled expression of heterologous genes governed by regulatory signals of proU. The pOSEX vectors carry the proU promoter, an upstream region required for high-level expression, and part of the first structural gene (proV), which acts as a silencer and is necessary to maintain low-level expression in low osmolarity media. An extended multiple cloning site (MCS) positioned at the 3' end of proV' permits the cloning of heterologous genes into the pOSEX plasmids, and efficient transcription terminators derived from the rrnB operon prevent deleterious read-through transcription into the vector portion. The properties of the pOSEX expression vectors were tested by positioning a promoterless lacZ (encoding beta-galactosidase) gene from E. coli and the gcdA (encoding carboxytransferase) gene from the Gram+ bacterium Acidaminococcus fermentans under the control of the proU regulatory region. Efficient, osmo-regulated and finely tuned expression of both lacZ and gcdA was achieved, and the amount of beta-galactosidase and carboxytransferase synthesized were simply controlled by adjusting the osmolarity of the growth medium with various concentrations of NaCl.

N-terminal protein sequence analysis of the rabbit erythrocyte lactate transporter suggests identity with the cloned monocarboxylate transport protein MCT1

An improved purification for the rabbit erythrocyte lactate transporter, using aminoethyl-Sepharose chromatography, is described. The process of purification of the 40-50 kDa transporter, labelled with 4,4'-diisothiocyanostilbene-2,2'-disulphonate (DIDS), was followed by Western blotting with anti-DIDS antibodies [Poole, R. C. and Halestrap, A. P. (1992) Biochem. J. 283, 855-862]. Fractions highly-enriched in transporter were further purified by SDS/PAGE and the 40-50 kDa DIDS-labelled polypeptide was subjected to N-terminal protein sequencing. This analysis identified the first 16 amino acids of the protein. With the exception of one conservative substitution, this protein sequence is identical to the N-terminal protein sequence predicted from a cDNA isolated from Chinese hamster ovary cells that encode a monocarboxylate transporter, MCT1 [Kim Garcia, C., Goldstein, J. L., Pathak, R. K., Anderson, R. G. W. and Brown, M. S. (1994) Cell 76, 865-873]. This observation, along with similarities in functional properties, leads us to conclude that lactate transport in rabbit erythrocytes is mediated by the MCT1 monocarboxylate transporter isoform.

The Escherichia coli proU promoter element and its contribution to osmotically signaled transcription activation

The proU operon of Escherichia coli encodes a high-affinity glycine betaine transport system which is osmotically inducible and enables the organism to recover from the deleterious effects of hyperosmotic shock. Regulation occurs at the transcriptional level. KMnO4 footprinting showed that the preponderance of transcription initiated at a single primary promoter region and that proU transcription activation did not occur differentially at alternate promoters in response to various levels of salt shock. Mutational analysis confirmed the location of the primary promoter and identified an extended -10 region required for promoter activity. Specific nucleotides within the spacer, between position -10 and position -35, were important for maximal expression, but every mutant which retained transcriptional activity remained responsive to osmotic signals. A chromosomal 90-bp minimal promoter fragment fused to lacZ was not significantly osmotically inducible. However, transcription from this fragment was resistant to inhibition by salt shock. A mutation in osmZ, which encodes the DNA-binding protein H-NS, derepressed wild-type proU expression by sevenfold but did not alter expression from the minimal promoter. The current data support a model in which the role of the proU promoter is to function efficiently at high ionic strength while other cis-acting elements receive and respond to the osmotic signal.

The complete sequencing of a 24.6 kb segment of yeast chromosome XI identified the known loci URA1, SAC1 and TRP3, and revealed 6 new open reading frames including homologues to the threonine dehydratases, membrane transporters, hydantoinases and the phospholipase A2-activating protein

We report the entire sequence of a 26.4 kb segment of chromosome XI of Saccharomyces cerevisiae. Identification of the known loci URA1, TRP3 and SAC1 revealed a translocation compared to the genetic map. Additionally, six unknown open reading frames have been identified. One of them is similar to catabolic threonine dehydratases. Another one contains characteristic features of membrane transporters. A third one is homologous in half of its length to the prokaryotic hydantoinase HyuA and in the other half to hydatoinase HyuB. A fourth one is homologous to the mammalian phospholipase A2-activating protein. A fifth one, finally, is homologous to the hypothetical open reading frame YCR007C of chromosome III. The sequence has been deposited in the EMBL data library under Accession Number X75951.

Adaptation of Escherichia coli to high osmolarity environments: osmoregulation of the high-affinity glycine betaine transport system proU

A sudden increase in the osmolarity of the environment is highly detrimental to the growth and survival of Escherichia coli and Salmonella typhimurium since it triggers a rapid efflux of water from the cell, resulting in a decreased turgor. Changes in the external osmolarity must therefore be sensed by the microorganisms and this information must be converted into an adaptation process that aims at the restoration of turgor. The physiological reaction of the cell to the changing environmental condition is a highly coordinated process. Loss of turgor triggers a rapid influx of K+ ions into the cell via specific transporters and the concomitant synthesis of counterions, such as glutamate. The increased intracellular concentration of K(+)-glutamate allows the adaptation of the cell to environments of moderately high osmolarities. At high osmolarity, K(+)-glutamate is insufficient to ensure cell growth, and the bacteria therefore replace the accumulated K+ ions with compounds that are less deleterious for the cell's physiology. These compatible solutes include polyoles such as trehalose, amino acids such as proline, and methyl-amines such as glycine betaine. One of the most important compatible solutes for bacteria is glycine betaine. This potent osmoprotectant is widespread in nature, and its intracellular accumulation is achieved through uptake from the environment or synthesis from its precursor choline. In this overview, we discuss the properties of the high-affinity glycine betaine transport system ProU and the osmotic regulation of its structural genes.

Sequencing of a 13.2 kb segment next to the left telomere of yeast chromosome XI revealed five open reading frames and recent recombination events with the right arms of chromosomes III and V

We report the entire sequence of a 13.2 kb segment next to the left telomere of chromosome XI of Saccharomyces cerevisiae. A 1.2 kb fragment near one end is 91% homologous to the right arm of chromosome III and 0.7 kb of that are 77% homologous to the right arm of chromosome V. Five open reading frames are included in the sequenced segment. Two of them are almost identical to the known YCR104W and YCR103C hypothetical proteins of chromosome III. A third one contains a region homologous to the Zn (2)-Cys (6) binuclear cluster pattern of fungal transcriptional activators. The fourth one, part of which is similar to the mammalian putative transporter of mevalonate, has the structure of membrane transporters. The fifth one is similar to yeast ferric reductase.

Nucleotide sequence and 3'-end deletion studies indicate that the K(+)-uptake protein kup from Escherichia coli is composed of a hydrophobic core linked to a large and partially essential hydrophilic C terminus

The kup (formerly trkD) gene from Escherichia coli encodes a minor K(+)-uptake system. The gene is located just upstream of the rbsDACBK operon at 84.5 min on the chromosome and is transcribed clockwise. kup codes for a 69-kDa protein, which may be composed of two domains. The first 440 amino acid residues appear to form an integral membrane protein that might traverse the cell membrane 12 times. The C-terminal 182 amino acid residues are predicted to form a hydrophilic domain located at the cytoplasmic side of the membrane. Deletion studies from the 3' end of kup showed that removal of almost the complete hydrophilic domain of the protein reduced, but did not abolish, K(+)-uptake activity.

The 12-transmembrane helix transporters

From the hydropathic profiles of their amino acid sequences many transport proteins are conceived to comprise 12-transmembrane alpha-helices. In only a few examples, however, is there genetical and/or biochemical evidence to support the 12-helix structure or illuminate the molecular mechanism of the transport process. A number of these transport proteins occur in evolutionarily related families, and sometimes superfamilies, indicating divergent evolution of the 12-helix structure. Other individual members or families of transport proteins are sufficiently different in amino acid sequence for their evolution to have taken place by convergence from independent ancestral origins.

Spontaneous pmrA mutants of Salmonella typhimurium LT2 define a new two-component regulatory system with a possible role in virulence

We isolated spontaneous mutations (pmrA) in the smooth strain Salmonella typhimurium LT2 that show increased resistance to the cationic antibacterial proteins of human neutrophils and to the drug polymyxin B. The mutation in one strain, JKS5, maps to 93 min on the S. typhimurium chromosome, near the proP gene and the melAB operon. The mutation, designated pmrA505, confers a 1,000-fold increase in resistance to polymyxin B and a 2- to 4-fold increase in resistance to neutrophil proteins. We cloned both the pmrA505 and pmrA+ alleles and found that the pmrA+ gene is partially dominant over pmrA505. DNA sequence analysis of the pmrA505 clone revealed three open reading frames (ORFs). The deduced amino acid sequences indicated that ORF1 encodes a 548-amino-acid (aa) protein with a putative membrane-spanning domain and no significant homology to any known protein. ORF2 and ORF3, which encode 222- and 356-aa proteins, respectively, show strong homology with the OmpR-EnvZ family of two-component regulatory systems. ORF2 showed homology with a number of response regulators, including OmpR and PhoP, while ORF3 showed homology to histidine kinase-sensor proteins EnvZ and PhoR. Genetic analysis of the cloned genes suggested that ORF2 contained the pmrA505 mutation. Comparison of the pmrA505 and pmrA+ ORF2 DNA sequences revealed a single G-A transition, which would result in a His-to-Arg substitution at position 81 in the ORF2 mutant protein. We therefore designate ORF2 PmrA and ORF3 PmrB. The function of ORF1 is unknown.