Cloning of extracellular DNase and construction of a DNase-negative strain of Vibrio cholerae

Nuclease activity: an exploitable biomarker in bacterial infections

INTRODUCTION

In the increasingly challenging field of clinical microbiology, diagnosis is a cornerstone whose accuracy and timing are crucial for the successful management, therapy, and outcome of infectious diseases. Currently employed biomarkers of infectious diseases define the scope and limitations of diagnostic techniques. As such, expanding the biomarker catalog is crucial to address unmet needs and bring about novel diagnostic functionalities and applications.

AREAS COVERED

This review describes the extracellular nucleases of 15 relevant bacterial pathogens and discusses the potential use of nuclease activity as a diagnostic biomarker. Articles were searched for in PubMed using terms: "nuclease", "bacteria", "nuclease activity" or "biomarker". For overview sections, original and review articles between 2000 and 2019 were searched for using terms: "infections", "diagnosis", "bacterial", "burden", "challenges". Informative articles were selected.

EXPERT OPINION

Using the catalytic activity of nucleases offers new possibilities compared to established biomarkers. Nucleic acid activatable reporters in combination with different transduction platforms and delivery methods can be used to detect disease-associated nuclease activity patterns in vitro and in vivo for prognostic and diagnostic applications. Even when these patterns are not obvious or of unknown etiology, screening platforms could be used to identify new disease reporters.

Characterization of Vibrio cholerae's Extracellular Nuclease Xds

The Gram-negative bacterium Vibrio cholerae encodes two nucleases, Dns and Xds, which play a major role during the human pathogen's lifecycle. Dns and Xds control three-dimensional biofilm formation and bacterial detachment from biofilms via degradation of extracellular DNA and thus contribute to the environmental, inter-epidemic persistence of the pathogen. During intestinal colonization the enzymes help evade the innate immune response, and therefore promote survival by mediating escape from neutrophil extracellular traps. Xds has the additional function of degrading extracellular DNA down to nucleotides, which are an important nutrient source for V. cholerae. Thus, Xds is a key enzyme for survival fitness during distinct stages of the V. cholerae lifecycle and could be a potential therapeutic target. This study provides detailed information about the enzymatic properties of Xds using purified protein in combination with a real time nuclease activity assay. The data define an optimal buffer composition for Xds activity as 50 mM Tris/HCl pH 7, 100 mM NaCl, 10 mM MgCl2, and 20 mM CaCl2. Moreover, maximal activity was observed using substrate DNA with low GC content and ambient temperatures of 20-25°C. In silico analysis and homology modeling predicted an exonuclease domain in the C-terminal part of the protein. Biochemical analyses with truncated variants and point mutants of Xds confirm that the C-terminal region is sufficient for nuclease activity. We also find that residues D787 and H837 within the predicted exonuclease domain are key to formation of the catalytic center.

C-terminal processing of GlyGly-CTERM containing proteins by rhombosortase in Vibrio cholerae

Vibrio cholerae and a subset of other Gram-negative bacteria, including Acinetobacter baumannii, express proteins with a C-terminal tripartite domain called GlyGly-CTERM, which consists of a motif rich in glycines and serines, followed by a hydrophobic region and positively charged residues. Here we show that VesB, a V. cholerae serine protease, requires the GlyGly-CTERM domain, the intramembrane rhomboid-like protease rhombosortase, and the type II secretion system (T2SS) for localization at the cell surface. VesB is cleaved by rhombosortase to expose the second glycine residue of the GlyGly-CTERM motif, which is then conjugated to a glycerophosphoethanolamine-containing moiety prior to engagement with the T2SS and outer membrane translocation. In support of this, VesB accumulates intracellularly in the absence of the T2SS, and surface-associated VesB activity is no longer detected when the rhombosortase gene is inactivated. In turn, when VesB is expressed without an intact GlyGly-CTERM domain, VesB is released to the extracellular milieu by the T2SS and does not accumulate on the cell surface. Collectively, our findings suggest that the posttranslational modification of the GlyGly-CTERM domain is essential for cell surface localization of VesB and other proteins expressed with this tripartite extension.

The Type II secretion system delivers matrix proteins for biofilm formation by Vibrio cholerae

Gram-negative bacteria have evolved several highly dedicated pathways for extracellular protein secretion, including the type II secretion (T2S) system. Since substrates secreted via the T2S system include both virulence factors and degradative enzymes, this secretion system is considered a major survival mechanism for pathogenic and environmental species. Previous analyses revealed that the T2S system mediates the export of ≥ 20 proteins in Vibrio cholerae, a human pathogen that is indigenous to the marine environment. Here we demonstrate a new role in biofilm formation for the V. cholerae T2S system, since wild-type V. cholerae was found to secrete the biofilm matrix proteins RbmC, RbmA, and Bap1 into the culture supernatant, while an isogenic T2S mutant could not. In agreement with this finding, the level of biofilm formation in a static microtiter assay was diminished in T2S mutants. Moreover, inactivation of the T2S system in a rugose V. cholerae strain prevented the development of colony corrugation and pellicle formation at the air-liquid interface. In contrast, extracellular secretion of the exopolysaccharide VPS, an essential component of the biofilm matrix, remained unaffected in the T2S mutants. Our results indicate that the T2S system provides a mechanism for the delivery of extracellular matrix proteins known to be important for biofilm formation by V. cholerae. Because the T2S system contributes to the pathogenicity of V. cholerae by secreting proteins such as cholera toxin and biofilm matrix proteins, elucidation of the molecular mechanism of T2S has the potential to lead to the development of novel preventions and therapies.

Identification of in vivo regulators of the Vibrio cholerae xds gene using a high-throughput genetic selection

Vibrio cholerae, the causative agent of cholera, remains a threat to public health in areas with inadequate sanitation. As a waterborne pathogen, V. cholerae moves between two dissimilar environments, aquatic reservoirs and the intestinal tract of humans. Accordingly, this pathogen undergoes adaptive shifts in gene expression throughout the different stages of its lifecycle. One particular gene, xds, encodes a secreted exonuclease that was previously identified as being induced during infection. Here we sought to identify regulators responsible for the in vivo-specific induction of xds. A transcriptional fusion of xds to two consecutive antibiotic resistance genes was used to select transposon mutants that had inserted within or adjacent to regulatory genes and thereby caused increased expression of the xds fusion under non-inducing conditions. Large pools of selected insertion sites were sequenced in a high throughput manner using Tn-seq to identify potential mechanisms of xds regulation. Our selection identified the two-component system PhoB/R as the dominant activator of xds expression. In vitro validation confirmed that PhoB, a protein which is only active during phosphate limitation, was responsible for xds activation. Using xds expression as a biosensor of the extracellular phosphate level, we observed that the mouse small intestine is a phosphate-limited environment.

Cues and regulatory pathways involved in natural competence and transformation in pathogenic and environmental Gram-negative bacteria

Bacterial genomics is flourishing, as whole-genome sequencing has become affordable, readily available and rapid. As a result, it has become clear how frequently horizontal gene transfer (HGT) occurs in bacteria. The potential implications are highly significant because HGT contributes to several processes, including the spread of antibiotic-resistance cassettes, the distribution of toxin-encoding phages and the transfer of pathogenicity islands. Three modes of HGT are recognized in bacteria: conjugation, transduction and natural transformation. In contrast to the first two mechanisms, natural competence for transformation does not rely on mobile genetic elements but is driven solely by a developmental programme in the acceptor bacterium. Once the bacterium becomes competent, it is able to take up DNA from the environment and to incorporate the newly acquired DNA into its own chromosome. The initiation and duration of competence differ significantly among bacteria. In this review, we outline the latest data on representative naturally transformable Gram-negative bacteria and how their competence windows differ. We also summarize how environmental cues contribute to the initiation of competence in a subset of naturally transformable Gram-negative bacteria and how the complexity of the niche might dictate the fine-tuning of the competence window.

Extracellular nucleases and extracellular DNA play important roles in Vibrio cholerae biofilm formation

Biofilms are a preferred mode of survival for many microorganisms including Vibrio cholerae, the causative agent of the severe secretory diarrhoeal disease cholera. The ability of the facultative human pathogen V. cholerae to form biofilms is a key factor for persistence in aquatic ecosystems and biofilms act as a source for new outbreaks. Thus, a better understanding of biofilm formation and transmission of V. cholerae is an important target to control the disease. So far the Vibrio exopolysaccharide was the only known constituent of the biofilm matrix. In this study we identify and characterize extracellular DNA as a component of the Vibrio biofilm matrix. Furthermore, we show that extracellular DNA is modulated and controlled by the two extracellular nucleases Dns and Xds. Our results indicate that extracellular DNA and the extracellular nucleases are involved in diverse processes including the development of a typical biofilm architecture, nutrient acquisition, detachment from biofilms and the colonization fitness of biofilm clumps after ingestion by the host. This study provides new insights into biofilm development and transmission of biofilm-derived V. cholerae.

Generation of Vibrio anguillarum ghost by coexpression of PhiX 174 lysis E gene and staphylococcal nuclease A gene

Vibrio anguillarum ghosts (VAG) were generated, for the first time, using a conjugation vector containing a ghost bacteria inducing cassette, pRK-lambdaP(R)-cI-Elysis, in which the expression of PhiX174 lysis gene E was controlled by the P ( R )/cI regulatory system of lambda phage. By scanning electron microscopy, holes ranging 80-200 nm in diameter were observed in the VAG. To avoid the presence of bacterial genomic DNA and an antibiotic resistance gene in the final VAG product, we constructed a new dual vector, pRK-lambdaP(R)-cI-E-SNA, containing the E-mediated lysis cassette and the staphylococcal nuclease A (SNA)-mediated DNA degradation cassette, and generated safety-enhanced VAG for use as a fish vaccine.

The extracellular nuclease Dns and its role in natural transformation of Vibrio cholerae

Free extracellular DNA is abundant in many aquatic environments. While much of this DNA will be degraded by nucleases secreted by the surrounding microbial community, some is available as transforming material that can be taken up by naturally competent bacteria. One such species is Vibrio cholerae, an autochthonous member of estuarine, riverine, and marine habitats and the causative agent of cholera, whose competence program is induced after colonization of chitin surfaces. In this study, we investigate how Vibrio cholerae's two extracellular nucleases, Xds and Dns, influence its natural transformability. We show that in the absence of Dns, transformation frequencies are significantly higher than in its presence. During growth on a chitin surface, an increase in transformation efficiency was found to correspond in time with increasing cell density and the repression of dns expression by the quorum-sensing regulator HapR. In contrast, at low cell density, the absence of HapR relieves dns repression, leading to the degradation of free DNA and to the abrogation of the transformation phenotype. Thus, as cell density increases, Vibrio cholerae undergoes a switch from nuclease-mediated degradation of extracellular DNA to the uptake of DNA by bacteria induced to a state of competence by chitin. Taken together, these results suggest the following model: nuclease production by low-density populations of V. cholerae might foster rapid growth by providing a source of nucleotides for the repletion of nucleotide pools. In contrast, the termination of nuclease production by static, high-density populations allows the uptake of intact DNA and coincides with a phase of potential genome diversification.

Use of lambda phage S and R gene products in an inducible lysis system for Vibrio cholerae- and Salmonella enterica serovar typhimurium-based DNA vaccine delivery systems

Novel methods for adapting DNA vaccine technology to the prevention of mucosal diseases are greatly needed. Here we show that regulated expression of phage lambda lysis genes S and R causes dramatic lysis of both Vibrio cholerae and Salmonella enterica serovar Typhimurium cells with concomitant release of plasmid DNA into the surrounding media. We also used single and double DNase mutant strains to show that secreted V. cholerae DNases can adversely affect the integrity of DNA molecules released upon lysis. These results suggest that incorporation of lambda SR-mediated lysis constructs and DNA stabilizing mutations into candidate live attenuated bacterial vaccines offers a promising approach for the development of effective mucosal DNA delivery vectors for humans.

General secretion pathway (eps) genes required for toxin secretion and outer membrane biogenesis in Vibrio cholerae

The general secretion pathway (GSP) of Vibrio cholerae is required for secretion of proteins including chitinase, enterotoxin, and protease through the outer membrane. In this study, we report the cloning and sequencing of a DNA fragment from V. cholerae, containing 12 open reading frames, epsC to -N, which are similar to GSP genes of Aeromonas, Erwinia, Klebsiella, Pseudomonas, and Xanthomonas spp. In addition to the two previously described genes, epsE and epsM (M. Sandkvist, V. Morales, and M. Bagdasarian, Gene 123: 81-86, 1993; L. J. Overbye, M. Sandkvist, and M. Bagdasarian, Gene 132:101-106, 1993), it is shown here that epsC, epsF, epsG, and epsL also encode proteins essential for GSP function. Mutations in the eps genes result in aberrant outer membrane protein profiles, which indicates that the GSP, or at least some of its components, is required not only for secretion of soluble proteins but also for proper outer membrane assembly. Several of the Eps proteins have been identified by use of the T7 polymerase-promoter system in Escherichia coli. One of them, a pilin-like protein, EpsG, was analyzed also in V. cholerae and found to migrate as two bands on polyacrylamide gels, suggesting that in this organism it might be processed or otherwise modified by a prepilin peptidase. We believe that TcpJ prepilin peptidase, which processes the subunit of the toxin-coregulated pilus, TcpA, is not involved in this event. This is supported by the observations that apparent processing of EpsG occurs in a tcpJ mutant of V. cholerae and that, when coexpressed in E. coli, TcpJ cannot process EpsG although the PilD peptidase from Neisseria gonorrhoeae can.

Secreted enzymes of Aeromonas

A hallmark characteristic of species of Aeromonas is their ability to secrete a wide variety of enzymes associated with pathogenicity and environmental adaptability. Among the most intensively studied are beta-lactamases, lipases, hemolytic enterotoxins, proteases, chitinases, nucleases and amylases. Multiple copies of genes encoding each type of enzyme provide additional biological diversity. Except for the chitinases, these multiple copies show little evolutionary relatedness at the DNA level and only limited similarity at the protein level. Indeed a number of the genes, such as nuclease H of A. hydrophila, have no similarity to known prokaryotic or eukaryotic sequences. The challenge is to determine how these genes evolved, where they originated and why Aeromonas possesses them in such abundance and variety.

Cloning, sequencing, and characterization of the nucH gene encoding an extracellular nuclease from Aeromonas hydrophila JMP636

An Escherichia coli clone expressing activity on DNase agar was obtained by cloning chromosomal DNA of Aeromonas hydrophila JMP636 into plasmid pUC19. Examination (of the clone's nuclease activity on a sodium dodecyl sulfate (SDS)-polyacrylamide gel containing DNA as a substrate revealed an activity band at approximately 100 kDa. Subsequently, subcloning localized the gene, designated nucH, to a 3.6-kb DNA fragment (pJP9521). Southern blotting of the nucH gene against chromosomal DNA of JMP636 confirmed that it had originated from this strain and demonstrated that it was present in a single copy, although additional faint bands were also detected. Analysis of the subclone using in vivo transcription and translation revealed only a single polypeptide of approximately 110 kDa. Sequencing of pJP9521 predicted an open reading frame of 3,213 bp encoding a protein of 1,070 amino acids and having a molecular mass of 114 kDa. Comparison of the deduced nucleotide sequence and the NucH predicted protein sequence with relevant databases indicated that no known homologs have previously been identified. A signal sequence was predicted from these data, and cellular fractionation of a nucH clone in E. coli indicated that the protein was able to be processed to the periplasm. An activity similar in size was detected in an extracellular protein sample of JMP636, while inactivation of the nucH gene resulted in loss of this activity band. By native SDS-polyacrylamide gel electrophoresis, NucH substrate specificity, cofactor requirements, and sensitivity to denaturing agents were assessed.

Distinguishing between the extracellular DNases of Vibrio cholerae and development of a transformation system

Vibrio cholerae is known to secrete DNase(s) into the extracellular environment. These proteins have been thought to be responsible for the difficulties in transforming this organism. In this work we demonstrate that the dns and xds genes differ and that their products are solely responsible for the extracellular DNase activity. By site-directed mutagenesis, strains have been constructed which are mutant in one or both genes. These strains have been assessed for their ability to be transformed with plasmid DNA and for their virulence in the infant mouse cholera model. DNase-deficient mutants can be readily transformed and the product of dns appears to be the more significant barrier. No effect on virulence was observed with the mutants.

Transformation of Vibrio cholerae by plasmid DNA

The lack of an efficient transformation system in Vibrio cholerae was a handicap in the genetic manipulation of this important human pathogen. Since V. cholerae cells secrete DNases, this may interfere with the uptake of DNA. The present report describes the approaches taken for transforming V. cholerae cells with plasmid DNA, by overcoming this DNase barrier. The partial success of transforming DNase-negative mutants confirmed the role of DNase in the nontransformability of the wild-type cells. Successful transformation was carried out following removal of DNases from the periplasmic space. This was achieved by treating the cells with Mg2+ and Ca2+ ions to allow the DNase to be released, and then holding them under conditions where the remaining DNase activity was minimized before adding DNA to the competent cells. Transformation efficiencies of the order of 10(-5) per recipient cell were observed.

Genetic analysis in vibrio

Bacteria of the genus Vibrio are remarkably diverse, and until recently the methodology for genetic analysis consisted of a patchwork of different approaches, many of which were narrowly applicable to a single species. The invention of the recombinant DNA technology and the subsequent innovations in transposon mutagenesis and in transductive and conjugative gene transfer techniques have led to the development of very powerful and general strategies for genetic analysis of species of Vibrio. The striking synergy of combining recombinant DNA, transposon, and gene transfer methods is particularly evident in the construction of transposons which generate gene fusions and of broad host range plasmids which deliver transposons and mutated genes and which mobilize chromosomes. With such tools it should be possible to perform advanced genetic analysis on the many undomesticated species of Vibrio still to be explored.

Molecular cloning and physical and functional characterization of the Salmonella typhimurium and Salmonella typhi galactose utilization operons

The chromosomally encoded galactose utilization (gal) operons of Salmonella typhimurium and S. typhi were each cloned on similar 5.5-kilobase HindIII fragments into pBR322 and were identified by complementation of Gal- Escherichia coli strains. Restriction endonuclease analyses indicated that these Salmonellae operons share considerable homology, but some heterogeneities in restriction sites were observed. Subcloning and exonuclease mapping experiments showed that both operons have the same genetic organization as that established for the E. coli gal operon (i.e., 5' end, promoter, epimerase, transferase, kinase, and 3' end). Two gal operator regions (oE and oI) of S. typhimurium, identified by repressor titration in an E. coli superrepressor [galR(Sup)] mutant, were sequenced and found to flank the promoter region. This promoter region is identical to the -10 and -35 regions of the E. coli gal operon. Minicell studies demonstrated that the three gal structural genes of S. typhimurium encode separate polypeptides of 39 kilodaltons (kDa) (epimerase, 337 amino acids [aa's]), 41 kDa (transferase, 348 aa's), and 43 kDa (kinase, 380 aa's). Despite functional and organizational similarities, DNA sequence analysis revealed that the S. typhimurium gal genes show less than 70% homology to the E. coli gal operon. Because of codon degeneracy, the deduced amino acid sequences of these polypeptides are highly conserved (greater than 90% homology) as compared with those of the E. coli gal enzymes. These studies have defined basic genetic parameters of the gal genes of two medically important Salmonella species, and our findings support the hypothesized divergent evolution of E. coli and Salmonella spp. from a common ancestral parent bacterium.

Cloning of the genes for AF/R1 pili from rabbit enteroadherent Escherichia coli RDEC-1 and DNA sequence of the major structural subunit

AF/R1 pili on the surface of Escherichia coli RDEC-1 promote attachment of the bacteria to rabbit intestinal brush borders. In order to characterize AF/R1 pili and manipulate their expression, we cloned the genes necessary for AF/R1 expression; determined the size of proteins produced in minicells; located the gene encoding the major structural subunit, named AfrA; and determined the DNA sequence of afrA as well as the sequence of 700 additional nucleotides upstream of afrA. Two contiguous EcoRI fragments spanning 7.9 kilobases were cloned from the 86-megadalton plasmid of RDEC-1 into vector pUC19 to make plasmid pW1. Bacteria carrying pW1 produced AF/R1 pili that were recognized by AF/R1-specific antiserum and promoted adherence of bacteria to brush borders prepared from rabbit intestine. Proteins with a molecular weight of 17,000 (17K proteins), which was the size of AfrA, as well as 15K, 15.5K, 26K, 28K, and 80K proteins were detected in minicells carrying pW1. The gene afrA was located by using an oligonucleotide probe, and its DNA sequence was determined. The DNA sequence of 700 additional nucleotides upstream was determined because this sequence may be important in the regulation of AF/R1 expression.

Characterization of the hlyB gene and its role in the production of the El Tor haemolysin of Vibrio cholerae O1

El Tor strains of Vibrio cholerae are capable of producing a haemolysin which they actively secrete into the growth medium. This requires translation to produce the protein at the surface of the cytoplasmic membrane and translocation across this membrane, the periplasmic space and the outer membrane. The mechanism by which this occurs is poorly understood. In addition to the structural gene for the haemolysin (hlyA), we have cloned a second adjacent gene, hlyB. By site-directed mutagenesis, specific hlyB mutants have been constructed. These mutants are defective in the secretion of HlyA in the early to mid-exponential phase of growth and the haemolysin becomes trapped within the cell and is only released in stationary phase. Nucleotide sequence analysis and cell fractionations reveal HlyB to be a 60.3 kD putative outer membrane-associated protein.

Effects of DNase production, plasmid size, and restriction barriers on transformation of Vibrio cholerae by electroporation and osmotic shock

Attempts to transform wild type strains of V. cholerae with plasmid DNA by traditional osmotic shock methods were not successful. A mutant of V. cholerae that was deficient in extracellular DNase was transformed with plasmid DNA by osmotic shock, demonstrating directly that extracellular DNase is a major barrier to transformation of V. cholerae. Transformation of wild type and DNase-negative strains of V. cholerae was accomplished by electroporation. Efficiency of transformation by electroporation increased with field strength, decreased with plasmid size, and was relatively insensitive to changes in the electrolyte composition of the buffer as long as isotonic sucrose was present. Host-controlled modification/restriction systems also affected transformation efficiency in V. cholerae.

Characterization of invasion plasmid antigen genes (ipaBCD) from Shigella flexneri

The large invasion plasmid of Shigella flexneri M9OT-W was used to generate recombinant plasmids carrying the ipaA, -B, -C, and -D genes, whose products are associated with the entry of the bacteria into colonic epithelial cells. Complete DNA sequences of ipaB, -C, and -D were determined. The proteins predicted (62, 42, and 37 kDa, respectively) from the nucleotide sequences lack a signal-peptide sequence. Hydrophilic segments of the IpaB and IpaC proteins were found to overlap known epitopic domains of these membrane antigens. Analysis of total RNA demonstrated that temperature control of ipa gene expression occurs at the level of transcription. Multiple mRNA bands were detected by using ipa gene fragments as hybridization probes, and a putative transcript map for the ipa genes was constructed. Comparison of this map with the DNA sequence reveals a complex system of ipa gene regulation.

Extracellular proteins of Vibrio cholerae: nucleotide sequence of the structural gene (hlyA) for the haemolysin of the haemolytic El Tor strain 017 and characterization of the hlyA mutation in the non-haemolytic classical strain 569B

The EI T or haemolysin, product of hlyA, is exported from Vibrio cholerae as a Mr 80,000 protein which can be subsequently cleaved to give two proteins of Mr 65,000 and 15,000. Nucleotide sequence analysis has demonstrated that hlyA encodes a protein of Mr 82,250 with a potential 18-amino-acid signal sequence. The non-haemolytic classical strain 569B has been shown to have a structural gene defect rather than a defect in secretion. By non-reciprocal recombination it was possible to transfer this defect onto a plasmid and show that a truncated hlyA product of Mr 27,000 is made in Escherichia coli K-12 minicells. Nucleotide sequence analysis demonstrates an 11-base-pair deletion which would result in a Mr 26,940 protein probably loosely associated with the membrane.

Cloning of genes for production of Escherichia coli Shiga-like toxin type II

Genes controlling production of Shiga-like toxin type II (SLT-II) in Escherichia coli were cloned from the SLT-II-converting bacteriophage 933W and compared with the Shiga-like toxin type I (SLT-I) genes previously isolated and described from phage 933J. Subcloning analysis identified a region within the 4.9-kilobase EcoRI fragment of phage 933W that was associated with SLT-II production. Experiments with E. coli minicells containing these subclones demonstrated that the 4.9-kilobase EcoRI fragment encodes the structural genes for SLT-II. These experiments additionally showed the genetic organization of the SLT-II genes to be the same as that of the SLT-I genes, with the coding sequence for the large A subunit adjacent to that for the smaller B subunit. The mobilities of the SLT-II subunits in sodium dodecyl sulfate-polyacrylamide gels were slightly greater than those determined for the SLT-I subunits. Although apparent processing of the SLT-I subunits was observed with polymyxin B treatment of the labeled minicells, no processing of the SLT-II subunits was detected. Southern blot hybridization studies suggested that the DNA fragment carrying the SLT-II structural genes shares approximately 50 to 60% homology with the DNA of the SLT-I structural genes.

Genetics of Vibrio cholerae and its bacteriophages

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Extracellular proteins of Vibrio cholerae: molecular cloning, nucleotide sequence and characterization of the deoxyribonuclease (DNase) together with its periplasmic localization in Escherichia coli K-12

The gene encoding the extracellular DNase of Vibrio cholerae was cloned into Escherichia coli K-12. A maximal coding region of 1.2 kb and a minimal region of 0.6 kb were determined by transposon mutagenesis and deletion analysis. The nucleotide sequence of this region contained a single open reading frame of 690 bp corresponding to a protein of Mr 26,389 with a typical N-terminal signal sequence of 18 aa which, when removed, would give a mature protein of Mr 24,163. This is in good agreement with the size of 24 kDa, calculated directly by Coomassie blue staining following sodium dodecyl sulphate-polyacrylamide gel electrophoresis and indirectly via a DNA-hydrolysis assay. The protein is located in the periplasmic space of E. coli K-12 unlike in V. cholerae where it is excreted into the extracellular medium. The introduction of the DNase gene into a periplasmic (tolA) leaky mutant of E. coli K-12 facilitates the release of the protein, further confirming the periplasmic location.