Selection of mutations that alter the osmotic control of transcription of the Salmonella typhimurium proU operon

Mechanisms underlying the low-temperature adaptation of 17β-estradiol-degrading bacterial strain Rhodococcus sp. RCBS9: insights from physiological and transcriptomic analyses

The 17β-estradiol (E2)-degrading bacterium Rhodococcus sp.RCBS9 previously showed remarkable resistance to the combined stresses of low temperature and E2. In this study, physiological experiments and transcriptomic analysis were performed to investigate the mechanisms underlying the strain's low-temperature adaptation and briefly analyze how it maintains its ability to degrade E2 at low temperature. The results showed that the strain's signal transduction functions, adaptive changes in cell membrane and cell wall structure, gene repair functions, and synthesis of antioxidants and compatible solutes are key to its ability to adapt to low temperature. In addition, its stress proteins in response to low temperature were not typical cold shock proteins, but rather universal stress proteins (USPs) and heat shock proteins (HSPs), among others. The strain also upregulated biofilm production, transporter proteins for carbon source uptake, and proteins for fatty acid degradation to ensure energy generation. The strain's multiple stress responses work synergistically to resist low-temperature stress, ensuring its adaptability to low-temperature environments and ability to degrade E2. Finally, six genes related to survival at low temperature (identified in the transcriptome analysis) were expressed in E. coli BL21, and they were found to contribute to recombinant E. coli growth at low temperature.

Biosynthesis of Proline

Proline was among the last biosynthetic precursors to have its biosynthetic pathway unraveled. This review recapitulates the findings on the biosynthesis and transport of proline. Glutamyl kinase (GK) catalyzes the ATP-dependent phosphorylation of L-glutamic acid. Purification of γ-GK from Escherichia coli was facilitated by the expression of the proB and proA genes from a high-copy-number plasmid and the development of a specific coupled assay based on the NADPH-dependent reduction of GP by γ-glutamyl phosphate reductase (GPR). GPR catalyzes the NADPH-dependent reduction of GP to GSA. Site directed mutagenesis was used to identify residues that constitute the active site of E. coli GK. This analysis indicated that there is an overlap between the binding sites for glutamate and the allosteric inhibitor proline, suggesting that proline competes with the binding of glutamate. The review also summarizes the genes involved in the metabolism of proline in E. coli and Salmonella. Among the completed genomic sequences of Enterobacteriaceae, genes specifying all three proline biosynthetic enzymes can be discerned in E. coli, Shigella, Salmonella enterica, Serratia marcescens, Erwinia carotovora, Yersinia, Photorhabdus luminescens, and Sodalis glossinidius strain morsitans. The intracellular proline concentration increases with increasing external osmolality in proline-overproducing mutants. This apparent osmotic regulation of proline accumulation in the overproducing strains may be the result of increased retention or recapture of proline, achieved by osmotic stimulation of the ProP or ProU proline transport systems. A number of proline analogs can be incorporated into proteins in vivo or in vitro.

Timing of induction of osmotically controlled genes in Salmonella enterica Serovar Typhimurium, determined with quantitative real-time reverse transcription-PCR

The signals that control the transcription of osmoregulated genes are not understood satisfactorily. The "turgor control model" suggested that the primary osmoregulatory signal in Enterobacteriaceae is turgor loss, which induces the kdp K+ transport operon and activates the Trk K+ permease. The ensuing increase in cytoplasmic K+ concentration was proposed to be the signal that turns on all secondary responses, including the induction of the proU (proline-glycine betaine transport) operon. The "ionic strength model" proposed that the regulatory signal for all osmotically controlled responses is the increase in the cytoplasmic ionic strength or macromolecular crowding after an osmotic upshift. The assumption in the turgor control model that the induction of kdp is a primary response to osmotic shock predicts that this response should precede all secondary responses. Both models predict that the induction of all osmotically activated responses should be independent of the chemical nature of the solute used to impose osmotic stress. We tested these predictions by quantitative real-time reverse transcription-PCR analysis of the expression of six osmotically regulated genes in Salmonella enterica serovar Typhimurium. After shock with 0.3 M NaCl, proU was induced at 4 min, proP and rpoS were induced at 4 to 6 min, kdp was induced at 8 to 9 min, and otsB and ompC were induced at 10 to 12 min. After an equivalent osmotic shock with 0.6 M sucrose, proU was induced with kinetics similar to those seen with NaCl, but induction of kdp was reduced 150-fold in comparison to induction by NaCl. Our results are inconsistent with both the turgor control and the ionic strength control models.

The downstream regulatory element of the proU operon of Salmonella typhimurium inhibits open complex formation by RNA polymerase at a distance

The intracellular concentration of K(+)-glutamate, chromatin-associated proteins, and a downstream regulatory element (DRE) overlapping with the coding sequence, have been implicated in the regulation of the proU operon of Salmonella typhimurium. The basal expression of the proU operon is low, but it is rapidly induced when the bacteria are grown in media of high osmolarity (e.g. 0.3 M NaCl). It has previously been suggested that increased intracellular concentrations of K(+)-glutamate activate the proU promoter in response to increased extracellular osmolarity. We show here that the activation of the proU promoter by K(+)-glutamate in vitro is nonspecific, and the in vivo regulation cannot simply be mimicked in vitro. In vivo specificity requires both the chromatin-associated protein H-NS and the DRE; they are both needed to maintain repression of proU expression at low osmolarity. How H-NS and the DRE repress the proU promoter in vivo has so far been unclear. We show that, in vivo, the DRE acts at a distance to inhibit open complex formation at the proU promoter.

How is osmotic regulation of transcription of the Escherichia coli proU operon achieved? A review and a model

The proU operon in enterobacteria encodes a binding-protein-dependent transporter for the active uptake of glycine betaine and L-proline, and serves an adaptive role during growth of cells in hyperosmolar environments. Transcription of proU is induced 400-fold under these conditions, but the underlying signal transduction mechanisms are incompletely understood. Increased DNA supercoiling and activation by potassium glutamate have each been proposed in alternative models as mediators of proU osmoresponsivity. We review here the available experimental data on proU regulation, and in particular the roles for DNA supercoiling, potassium glutamate, histone-like proteins of the bacterial nucleoid, and alternative sigma factors of RNA polymerase in such regulation. We also propose a new unifying model, in which the pronounced osmotic regulation of proU expression is achieved through the additive effects of at least three separate mechanisms, each comprised of a cis element [two promoters P1 and P2, and negative-regulatory-element (NRE) downstream of both promoters] and distinct trans-acting factors that interact with it: stationary-phase sigma factor RpoS with P1, nucleoid proteins HU and IHF with P2, and nucleoid protein H-NS with the NRE. In this model, potassium glutamate may activate proU expression through each of the three mechanisms whereas DNA supercoiling has a very limited role, if any, in the osmotic induction of proU transcription. We also suggest that proU may be a virulence gene in the pathogenic enterobacteria.

Characterization of a chimeric proU operon in a subtilin-producing mutant of Bacillus subtilis 168

The ability to respond to osmotic stress by osmoregulation is common to virtually all living cells. Gram-negative bacteria such as Escherichia coli and Salmonella typhimurium can achieve osmotolerance by import of osmoprotectants such as proline and glycine betaine by an import system encoded in an operon called proU with genes for proteins ProV, ProW, and ProX. In this report, we describe the discovery of a proU-type locus in the gram-positive bacterium Bacillus subtilis. It contains four open reading frames (ProV, ProW, ProX, and ProZ) with homology to the gram-negative ProU proteins, with the B. subtilis ProV, ProW, and ProX proteins having sequence homologies of 35, 29, and 17%, respectively, to the E. coli proteins. The B. subtilis ProZ protein is similar to the ProW protein but is smaller and, accordingly, may fulfill a novel role in osmoprotection. The B. subtilis proU locus was discovered while exploring the chromosomal sequence upstream from the spa operon in B. subtilis LH45, which is a subtilin-producing mutant of B. subtilis 168. B. subtilis LH45 had been previously constructed by transformation of strain 168 with linear DNA from B. subtilis ATCC 6633 (W. Liu and J. N. Hansen, J. Bacteriol. 173:7387-7390, 1991). Hybridization experiments showed that LH45 resulted from recombination in a region of homology in the proV gene, so that the proU locus in LH45 is a chimera between strains 168 and 6633. Despite being a chimera, this proU locus was fully functional in its ability to confer osmotolerance when glycine betaine was available in the medium. Conversely, a mutant (LH45 deltaproU) in which most of the proU locus had been deleted grew poorly at high osmolarity in the presence of glycine betaine. We conclude that the proU-like locus in B. subtilis LH45 is a gram-positive counterpart of the proU locus in gram-negative bacteria and probably evolved prior to the evolutionary split of prokaryotes into gram-positive and gram-negative forms.

Fine-structure deletion analysis of the transcriptional silencer of the proU operon of Salmonella typhimurium

Transcriptional control of the osmotically regulated proU operon of Salmonella typhimurium is mediated in part by a transcriptional silencer downstream from the promoter (D.G. Overdier and L.N. Csonka, Proc. Natl. Acad. Sci. USA 89:3140-3144, 1992). We carried out a fine-structure deletion analysis to determine the structure and the position of the silencer, which demonstrated that this regulatory element is located between nucleotide positions +73 to +274 downstream from the transcription start site. The silencer appears to be made up of a number of components which have cumulative negative regulatory effects. Deletions or insertions of short nucleotide sequences (< 40 bp) between the proU promoter and the silencer do not disrupt repression exerted by the silencer, but long insertions (> or = 0.8 kbp) result in a high level of expression from the proU promoter, similar to that imparted by deletion of the entire silencer. The general DNA-binding protein H-NS is required for the full range of repression of the proU operon in media of low osmolality. Although in the presence of the silencer hns mutations increased basal expression from the proU promoter three- to sixfold, in the absence of the silencer they did not result in a substantial increase in basal expression from the proU promoter. Furthermore, deletion of the silencer in hns+ background was up to 10-fold more effective in increasing basal expression from the proU promoter than the hns mutations. These results indicate that osmotic control of the proU operon is dependent of some factor besides H-NS. We propose that the transcriptional regulation of this operon is effected in media of low osmolality by a protein which makes the promoter inaccessible to RNA polymerase by forming a complex containing the proU promoter and silencer.

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.

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.

Molecular characterization of the promoter of osmY, an rpoS-dependent gene

The osmY gene, which encodes a periplasmic protein with an apparent M(r) of 22,000, is induced by both osmotic and growth phase signals. We demonstrate here that osmY expression is regulated at the level of transcription and that transcription initiates 242 nucleotides upstream of the osmY open reading frame. Relative to the transcriptional start site, 5' deletions up to -36 did not inhibit osmY expression. 3' deletions that extended into the untranslated leader region affected the overall level of osmY::lacZ expression but did not affect inducibility. 5' and 3' deletions that extended past the transcriptional start region essentially abolished osmY expression, suggesting that there is a single promoter region. A putative promoter was identified, and its -10 region, TATATT, closely resembles the sigma 70 consensus -10 sequence, TATAAT. However, we show that osmY is not absolutely dependent on a functional sigma 70 for its expression. Since osmY expression does require rpoS (R. Hengge-Aronis, R. Lange, N. Henneberg, and D. Fischer, J. Bacteriol. 175:259-265, 1993), which encodes a stationary-phase sigma factor, sigma S (K. Tanaka, Y. Takayanagi, N. Fujita, A. Ishihama, and H. Takahashi, Proc. Natl. Acad. Sci. USA 90:3511-3515, 1993), E sigma S may be the form of RNA polymerase responsible for transcription of osmY.

The chromatin-associated protein H-NS interacts with curved DNA to influence DNA topology and gene expression

H-NS is an abundant structural component of bacterial chromatin and influences many cellular processes, including recombination, transposition, and transcription. We have studied the mechanism of action of H-NS at the osmotically regulated proU promoter. The interaction of H-NS with a curved DNA element located downstream of the proU promoter is required for normal regulation of expression. Heterologous curved sequences can replace the regulatory role of the proU curve. Hence, the luxAB and lacZ reporter genes, which differ in the presence or absence of a curve, can indicate very different patterns of transcription. H-NS interacts preferentially with these curved DNA elements in vitro. Furthermore, in vivo the interaction of H-NS with curved DNA participates in the control of plasmid linking number. The data suggest that H-NS-dependent changes in DNA topology play a role in the osmoregulation of proU expression.

osmY, a new hyperosmotically inducible gene, encodes a periplasmic protein in Escherichia coli

A new osmotically inducible gene in Escherichia coli, osmY, was induced 8- to 10-fold by hyperosmotic stress and 2- to 3-fold by growth in complex medium. The osmY gene product is a periplasmic protein which migrates with an apparent molecular mass of 22 kDa on sodium dodecyl sulfate-polyacrylamide gels. A genetic fusion to osmY was mapped to 99.3 min on the E. coli chromosome. The gene was cloned and sequenced, and an open reading frame was identified. The open reading frame encoded a precursor protein with a calculated molecular weight of 21,090 and a mature protein of 18,150 following signal peptide cleavage. Sequencing of the periplasmic OsmY protein confirmed the open reading frame and defined the signal peptide cleavage site as Ala-Glu. A mutation caused by the osmY::TnphoA genetic fusion resulted in slightly increased sensitivity to hyperosmotic stress.

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.

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.

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.

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.

Nucleotide sequence of the transcriptional control region of the osmotically regulated proU operon of Salmonella typhimurium and identification of the 5' endpoint of the proU mRNA

Southern blot analysis of 15 proU transposon insertions in Salmonella typhimurium indicated that this operon is at least 3 kilobase pairs in length. The nucleotide sequence of 1.5-kilobase-pair fragment that contains the transcriptional control region of the proU operon and the coding sequences specifying 290 amino acids of the first structural gene of the operon was determined. The predicted amino acid sequence of the product of this gene shows extensive similarity to the HisP, MalK, and other proteins that are inner membrane-associated components of binding protein-dependent transport systems. S1 mapping and primer extension analysis of the proU mRNAs revealed several species with different 5' ends. Two of these endpoints are sufficiently close to sequences that have weak similarities to the consensus -35 and -10 promoter sequences that they are likely to define two transcription start sites. However, we cannot rule out the possibility that some or all of the 5' endpoints detected arose as a result of the degradation of a longer mRNA. The expression of proU-lacZ operon fusions located on plasmids was normal in S. typhimurium regardless of the plasmid copy number. The sequences mediating normal, osmoregulated expression of the proU operon were shown by subcloning to be contained on an 815-base-pair fragment. A 350-base-pair subclone of this fragment placed onto a lacZ expression vector directed a high-level constitutive expression of beta-galactosidase, suggesting that there is a site for negative regulation in the proU transcriptional control region which has been deleted in the construction of this plasmid.

Physiological and genetic responses of bacteria to osmotic stress

The capacity of organisms to respond to fluctuations in their osmotic environments is an important physiological process that determines their abilities to thrive in a variety of habitats. The primary response of bacteria to exposure to a high osmotic environment is the accumulation of certain solutes, K+, glutamate, trehalose, proline, and glycinebetaine, at concentrations that are proportional to the osmolarity of the medium. The supposed function of these solutes is to maintain the osmolarity of the cytoplasm at a value greater than the osmolarity of the medium and thus provide turgor pressure within the cells. Accumulation of these metabolites is accomplished by de novo synthesis or by uptake from the medium. Production of proteins that mediate accumulation or uptake of these metabolites is under osmotic control. This review is an account of the processes that mediate adaptation of bacteria to changes in their osmotic environment.

In vitro reconstitution of osmoregulated expression of proU of Escherichia coli

Osmoregulated expression of proU has been reconstituted in a cell-free system. proU encodes an osmotically inducible, high-affinity transport system for the osmoprotectant glycine betaine in Escherichia coli. Previously, a proU-lacZ fusion gene had been cloned, resulting in plasmid pOS3. In vivo osmoregulation of this extrachromosomal proU-lacZ fusion gene at low copy number showed that the plasmid-encoded fusion contained all the necessary sequences in cis for correctly receiving osmoregulatory signals during induction by osmotic stress and repression by glycine betaine. Using a cell-free (S-30) extract, plasmid pOS3 was then used to program protein synthesis in vitro. The ionic compound potassium glutamate specifically stimulated proU-lacZ expression in a concentration-dependent manner. Potassium acetate also induced some proU expression, but other salts were ineffective, thereby ruling out ionic strength as the stimulatory signal. High concentrations of sucrose, trehalose, or glycine betaine did not induce proU expression in vitro either, eliminating osmolarity per se as the stimulus. Reconstitution in a cell-free system rules out osmoregulatory mechanisms that depend on turgor, trans-membrane signaling, or trans-acting regulators synthesized after osmotic upshock.

Proline porters effect the utilization of proline as nutrient or osmoprotectant for bacteria

Proline is utilized by all organisms as a protein constituent. It may also serve as a source of carbon, energy and nitrogen for growth or as an osmoprotectant. The molecular characteristics of the proline transport systems which mediate the multiple functions of proline in the Gram negative enteric bacteria, Escherichia coli and Salmonella typhimurium, are now becoming apparent. Recent research on those organisms has provided both protocols for the genetic and biochemical characterization of the enzymes mediating proline transport and molecular probes with which the degree of homology among the proline transport systems of archaebacteria, eubacteria and eukaryotes can be assessed. This review has provided a detailed summary of recent research on proline transport in E. coli and S. typhimurium; the properties of other organisms are cited primarily to illustrate the generality of those observations and to show where homologous proline transport systems might be expected to occur. The characteristics of proline transport in eukaryotic microorganisms have recently been reviewed (Horak, 1986).

A physiological role for DNA supercoiling in the osmotic regulation of gene expression in S. typhimurium and E. coli

The proU locus encodes an osmotically inducible glycine betaine transport system that is important in the adaptation to osmotic stress. We present evidence that DNA supercoiling plays a key role in the osmotic induction of proU transcription. An increase in extracellular osmolarity increases in vivo DNA supercoiling, and the expression of proU is highly sensitive to these changes. Furthermore, topA mutations can mimic an increase in osmolarity, facilitating proU expression even in media of low osmolarity in which it is not normally expressed. Selection for trans-acting mutations that affect proU expression has yielded only mutations that alter DNA supercoiling, either in topA or a new genetic locus, osmZ, which strongly influences in vivo supercoiling. Mutations in osmZ are highly pleiotropic, affecting expression of a variety of chromosomal genes including ompF, ompC, fimA, and the bgl operon, as well as increasing the frequency of site-specific DNA inversions that mediate fimbrial phase variation.