Inactivation of the spirochete recA gene results in a mutant with low viability and irregular nucleoid morphology

Creating a Library of Random Transposon Mutants in Leptospira

Generation of a random transposon mutant library is advantageous in Leptospira as site-directed mutagenesis remains a challenge, especially in pathogenic species. This procedure is typically completed by transformation of Leptospira with a Himar1 containing plasmid via conjugation with Escherichia coli as a donor cell. Here we describe the methodology to generate random transposon mutants in the saprophyte Leptospira biflexa via conjugation of plasmid pSW29T-TKS2 harbored in E. coli β2163. Determination of transposon insertion site by semi-random nested PCR will also be described. A similar methodology may be employed to generate Tn mutants of pathogenic Leptospira species.

Screening of a Leptospira biflexa mutant library to identify genes involved in ethidium bromide tolerance

Leptospira spp. are spirochete bacteria comprising both pathogenic and free-living species. The saprophyte L. biflexa is a model bacterium for studying leptospiral biology due to relative ease of culturing and genetic manipulation. In this study, we constructed a library of 4,996 random transposon mutants in L. biflexa. We screened the library for increased susceptibility to the DNA intercalating agent, ethidium bromide (EtBr), in order to identify genetic determinants that reduce L. biflexa susceptibility to antimicrobial agents. By phenotypic screening, using subinhibitory EtBr concentrations, we identified 29 genes that, when disrupted via transposon insertion, led to increased sensitivity of the bacteria to EtBr. At the functional level, these genes could be categorized by function as follows: regulation and signaling (n=11), transport (n=6), membrane structure (n=5), stress response (n=2), DNA damage repair (n=1), and other processes (n=3), while 1 gene had no predicted function. Genes involved in transport (including efflux pumps) and regulation (two-component systems, anti-sigma factor antagonists, etc.) were overrepresented, demonstrating that these genes are major contributors to EtBr tolerance. This finding suggests that transport genes which would prevent EtBr to enter the cell cytoplasm are critical for EtBr resistance. We identified genes required for the growth of L. biflexa in the presence of sublethal EtBr concentration and characterized their potential as antibiotic resistance determinants. This study will help to delineate mechanisms of adaptation to toxic compounds, as well as potential mechanisms of antibiotic resistance development in pathogenic L. interrogans.

Leptospira interrogans serovar copenhageni harbors two lexA genes involved in SOS response

Bacteria activate a regulatory network in response to the challenges imposed by DNA damage to genetic material, known as the SOS response. This system is regulated by the RecA recombinase and by the transcriptional repressor lexA. Leptospira interrogans is a pathogen capable of surviving in the environment for weeks, being exposed to a great variety of stress agents and yet retaining its ability to infect the host. This study aims to investigate the behavior of L. interrogans serovar Copenhageni after the stress induced by DNA damage. We show that L. interrogans serovar Copenhageni genome contains two genes encoding putative LexA proteins (lexA1 and lexA2) one of them being potentially acquired by lateral gene transfer. Both genes are induced after DNA damage, but the steady state levels of both LexA proteins drop, probably due to auto-proteolytic activity triggered in this condition. In addition, seven other genes were up-regulated following UV-C irradiation, recA, recN, dinP, and four genes encoding hypothetical proteins. This set of genes is potentially regulated by LexA1, as it showed binding to their promoter regions. All these regions contain degenerated sequences in relation to the previously described SOS box, TTTGN ₅CAAA. On the other hand, LexA2 was able to bind to the palindrome TTGTAN₁₀TACAA, found in its own promoter region, but not in the others. Therefore, the L. interrogans serovar Copenhageni SOS regulon may be even more complex, as a result of LexA1 and LexA2 binding to divergent motifs. New possibilities for DNA damage response in Leptospira are expected, with potential influence in other biological responses such as virulence.

Positive regulation of Leptospira interrogans kdp expression by KdpE as Demonstrated with a novel β-galactosidase reporter in Leptospira biflexa

Leptospirosis is a potentially deadly zoonotic disease that afflicts humans and animals. Leptospira interrogans, the predominant agent of leptospirosis, encounters diverse conditions as it proceeds through its life cycle, which includes stages inside and outside the host. Unfortunately, the number of genetic tools available for examining the regulation of gene expression in L. interrogans is limited. Consequently, little is known about the genetic circuits that control gene expression in Leptospira. To better understand the regulation of leptospiral gene expression, the L. interrogans kdp locus, encoding homologs of the P-type ATPase KdpABC potassium transporter with their KdpD sensors and KdpE response regulators, was selected for analysis. We showed that a kdpE mutation in L. interrogans prevented the increase in kdpABC mRNA levels observed in the wild-type L. interrogans strain when external potassium levels were low. To confirm that KdpE was a positive regulator of kdpABC transcription, we developed a novel approach for constructing chromosomal genetic fusions to the endogenous bgaL (β-galactosidase) gene of the nonpathogen Leptospira biflexa. We demonstrated positive regulation of a kdpA'-bgaL fusion in L. biflexa by the L. interrogans KdpE response regulator. A control lipL32'-bgaL fusion was not regulated by KdpE. These results demonstrate the utility of genetic fusions to the bgaL gene of L. biflexa for examining leptospiral gene regulation.

Three-dimensional structures of pathogenic and saprophytic Leptospira species revealed by cryo-electron tomography

Leptospira interrogans is the primary causative agent of the most widespread zoonotic disease, leptospirosis. An in-depth structural characterization of L. interrogans is needed to understand its biology and pathogenesis. In this study, cryo-electron tomography (cryo-ET) was used to compare pathogenic and saprophytic species and examine the unique morphological features of this group of bacteria. Specifically, our study revealed a structural difference between the cell envelopes of L. interrogans and Leptospira biflexa involving variations in the lipopolysaccharide (LPS) layer. Through cryo-ET and subvolume averaging, we determined the first three-dimensional (3-D) structure of the flagellar motor of leptospira, with novel features in the flagellar C ring, export apparatus, and stator. Together with direct visualization of chemoreceptor arrays, DNA packing, periplasmic filaments, spherical cytoplasmic bodies, and a unique "cap" at the cell end, this report provides structural insights into these fascinating Leptospira species.

Frequent mutations within the genomic magnetosome island of Magnetospirillum gryphiswaldense are mediated by RecA

Genes for magnetosome formation in magnetotactic bacteria are clustered in large genomic magnetosome islands (MAI). Spontaneous deletions and rearrangements were frequently observed within these regions upon metabolic stress. This instability was speculated to be due to RecA-dependent homologous recombination between the numerous sequence repeats present within the MAI. Here we show that a RecA-deficient strain of Magnetospirillum gryphiswaldense (IK-1) no longer exhibits genetic instability of magnetosome formation. Strain IK-1 displayed higher sensitivity to oxygen and UV irradiation. Furthermore, the lack of RecA abolished allelic exchange in the mutant. Cells of strain IK-1 displayed a slightly altered (i.e., more elongated) morphology, whereas the absence of RecA did not affect the ability to synthesize wild-type-like magnetosomes. Our data provide evidence that the observed genetic instability of magnetosome formation in the wild type is due predominantly to RecA-mediated recombination. In addition, increased genetic stability could make strain IK-1 a useful tool for the expression of genes and further genetic engineering, as well as for biotechnological production of bacterial magnetosomes.

Leptospira as an emerging pathogen: a review of its biology, pathogenesis and host immune responses

Leptospirosis, the most widespread zoonosis in the world, is an emerging public health problem, particularly in large urban centers of developing countries. Several pathogenic species of the genus Leptospira can cause a wide range of clinical manifestations, from a mild, flu-like illness to a severe disease form characterized by multiorgan system complications leading to death. However, the mechanisms of pathogenesis of Leptospira are largely unknown. This article will address the animal models of acute and chronic leptospire infections, and the recent developments in the genetic manipulation of the bacteria, which facilitate the identification of virulence factors involved in pathogenesis and the assessment of their potential values in the control and prevention of leptospirosis.

Genetic manipulation of Leptospira biflexa

The genus Leptospira belongs to the order Spirochaetales and is composed of both saprophytic and pathogenic members, such as Leptospira biflexa and L. interrogans, respectively. A major factor contributing to our ignorance of spirochetal biology has been the lack of methods available for genetic analysis of these organisms. In recent years, an E. coli-L. biflexa shuttle vector has been constructed and a system for targeted mutagenesis and random transposon mutagenesis of the saprophyte L. biflexa has been developed. These studies enable the use of L. biflexa as a model bacterium among spirochetes.

Targeted mutagenesis in pathogenic Leptospira species: disruption of the LigB gene does not affect virulence in animal models of leptospirosis

The pathogenic mechanisms of Leptospira interrogans, the causal agent of leptospirosis, remain largely unknown. This is mainly due to the lack of tools for genetically manipulating pathogenic Leptospira species. Thus, homologous recombination between introduced DNA and the corresponding chromosomal locus has never been demonstrated for this pathogen. Leptospiral immunoglobulin-like repeat (Lig) proteins were previously identified as putative Leptospira virulence factors. In this study, a ligB mutant was constructed by allelic exchange in L. interrogans; in this mutant a spectinomycin resistance (Spc(r)) gene replaced a portion of the ligB coding sequence. Gene disruption was confirmed by PCR, immunoblot analysis, and immunofluorescence studies. The ligB mutant did not show decrease virulence compared to the wild-type strain in the hamster model of leptospirosis. In addition, inoculation of rats with the ligB mutant induced persistent colonization of the kidneys. Finally, LigB was not required to mediate bacterial adherence to cultured cells. Taken together, our data provide the first evidence of site-directed homologous recombination in pathogenic Leptospira species. Furthermore, our data suggest that LigB does not play a major role in dissemination of the pathogen in the host and in the development of acute disease manifestations or persistent renal colonization.

Borrelia burgdorferi vlsE antigenic variation is not mediated by RecA

RecA is a key protein linking genetic recombination to DNA replication and repair in bacteria. Previous functional characterization of Borrelia burgdorferi RecA indicated that the protein is mainly involved in genetic recombination rather than DNA repair. Genetic recombination may play a role in B. burgdorferi persistence by generation of antigenic variation. We report here the isolation of a recA null mutant in an infectious B. burgdorferi strain. Comparison of the in vitro growth characteristics of the mutant with those of the wild-type strain under various conditions showed no significant differences. While the RecA mutant was moderately more sensitive to UV irradiation and mitomycin C than the wild-type strain, the lack of RecA abolished allelic exchange in the mutant. Absence of RecA did not affect the ability of the mutant to infect mice. However, the RecA mutant was attenuated for joint infection in competitive-infection assays with the wild-type strain. vlsE sequence variation in mice was observed in both wild-type and RecA mutant spirochetes, indicating that the mechanism of antigenic variation is not homologous genetic recombination.

Leptospirosis: pathogenesis, immunity, and diagnosis

PURPOSE OF REVIEW

Leptospirosis is among the most important zoonotic diseases worldwide. Completion of the genomic sequences of leptospires has facilitated advances in diagnosis and prevention of the disease, and yielded insight into its pathogenesis. This article reviews this research, emphasizing recent progress.

RECENT FINDINGS

Leptospirosis is caused by a group of highly invasive spiral bacteria (spirochetes) that can infect both people and animals. Spirochetes can survive in the environment and host, and therefore outer membrane and secretory proteins that interact with the host are of considerable interest in leptospire research. The genetic approach to studying pathogenesis is hindered by fastidious growth of pathogenic leptospires. Integrated genomic and proteomic approaches, however, have yielded enhanced understanding of the pathogenesis of leptospirosis. Furthermore, studies of innate immune response to the organism have enhanced our understanding of host susceptibility and resistance to infection. In-silico analysis and high-throughput cloning and expression have had major impacts on efforts to develop vaccine candidates and diagnostic reagents.

SUMMARY

In the future, we must effectively utilize the wealth of genetic information to combat the disease. More studies into genetics, immune mechanisms that may be exploited to prevent leptospirosis, and pathogenesis of the disease are necessary.

Comparative and functional genomic analyses of iron transport and regulation in Leptospira spp

The spirochetes of the Leptospira genus contain saprophytic and pathogenic members, the latter being responsible for leptospirosis. Despite the recent sequencing of the genome of the pathogen L. interrogans, the slow growth of these bacteria, their virulence in humans, and a lack of genetic tools make it difficult to work with these pathogens. In contrast, the development of numerous genetic tools for the saprophyte L. biflexa enables its use as a model bacterium. Leptospira spp. require iron for growth. In this work, we show that Leptospira spp. can acquire iron from different sources, including siderophores. A comparative genome analysis of iron uptake systems and their regulation in the saprophyte L. biflexa and the pathogen L. interrogans is presented in this study. Our data indicated that, for instance, L. biflexa and L. interrogans contain 8 and 12 genes, respectively, whose products share homology with proteins that have been shown to be TonB-dependent receptors. We show that some genes involved in iron uptake were differentially expressed in response to iron. In addition, we were able to disrupt several putative genes involved in iron acquisition systems or iron regulation in L. biflexa. Comparative genomics, in combination with gene inactivation, gives us significant functional information on iron homeostasis in Leptospira spp.

Borrelia burgdorferi ftsZ plays a role in cell division

ftsZ is essential for cell division in many microorganisms. In Escherichia coli and Bacillus subtilis, FtsZ plays a role in ring formation at the leading edge of the cell division septum. An ftsZ homologue is present in the Borrelia burgdorferi genome (ftsZ(Bbu)). Its gene product (FtsZ(Bbu)) is strongly homologous to other bacterial FtsZ proteins, but its function has not been established. Because loss-of-function mutants of ftsZ(Bbu) might be lethal, the tetR/tetO system was adapted for regulated control of this gene in B. burgdorferi. Sixty-two nucleotides of an ftsZ(Bbu) antisense DNA sequence under the control of a tetracycline-responsive modified hybrid borrelial promoter were cloned into pKFSS1. This construct was electroporated into a B. burgdorferi host strain carrying a chromosomally located tetR under the control of the B. burgdorferi flaB promoter. After induction by anhydrotetracycline, expression of antisense ftsZ RNA resulted in generation of filamentous B. burgdorferi that were unable to divide and grew more slowly than uninduced cells. To determine whether FtsZ(Bbu) could interfere with the function of E. coli FtsZ, ftsZ(Bbu) was amplified from chromosomal DNA and placed under the control of the tetracycline-regulated hybrid promoter. After introduction of the construct into E. coli and induction with anhydrotetracycline, overexpression of ftsZ(Bbu) generated a filamentous phenotype. This suggested interference of ftsZ(Bbu) with E. coli FtsZ function and confirmed the role of ftsZ(Bbu) in cell division. This is the first report of the generation of a B. burgdorferi conditional lethal mutant equivalent by tetracycline-controlled expression of antisense RNA.

A homozygous recA mutant of Synechocystis PCC6803: construction strategy and characteristics eliciting a novel RecA independent UVC resistance in dark

We report here the construction of a homozygous recA460::cam insertion mutant of Synechocystis sp. PCC 6803 that may be useful for plant molecular genetics by providing a plant like host free of interference from homologous recombination. The homozygous recA460::cam mutant is highly sensitive to UVC under both photoreactivating and non-photoreactivating conditions compared to the wild type (WT). The liquid culture of the mutant growing in approximately 800 lx accumulates nonviable cells to the tune of 86% as estimated by colony counts on plates incubated at the same temperature and light intensity. The generation time of recA mutant in standard light intensity (2,500 lx) increases to 50 h compared to 28 h in lower light intensity (approximately 800 lx) that was used for selection, thus explaining the earlier failures to obtain a homozygous recA mutant. The WT, in contrast, grows at faster rate (23 h generation time) in standard light intensity compared to that at approximately 800 lx (26 h). The Synechocystis RecA protein supports homologous recombination during conjugation in recA (-) mutant of Escherichia coli, but not the SOS response as measured by UV sensitivity. It is suggested that using this homozygous recA460::cam mutant, investigations can now be extended to dissect the network of DNA repair pathways involved in housekeeping activities that may be more active in cyanobacteria than in heterotrophs. Using this mutant for the first time we provide a genetic evidence of a mechanism independent of RecA that causes enhanced UVC resistance on light to dark transition.

Random insertional mutagenesis of Leptospira interrogans, the agent of leptospirosis, using a mariner transposon

The recent availability of the complete genome sequences of Leptospira interrogans, the agent of leptospirosis, has allowed the identification of several putative virulence factors. However, to our knowledge, attempts to carry out gene transfer in pathogenic Leptospira spp. have failed so far. In this study, we show that the Himar1 mariner transposon permits random mutagenesis in the pathogen L. interrogans. We have identified genes that have been interrupted by Himar1 insertion in 35 L. interrogans mutants. This approach of transposon mutagenesis will be useful for understanding the spirochetal physiology and the pathogenic mechanisms of Leptospira, which remain largely unknown.

Isolation and characterization of FecA- and FeoB-mediated iron acquisition systems of the spirochete Leptospira biflexa by random insertional mutagenesis

The specific mechanisms by which Leptospira spp. acquire iron from their ecological niches are unknown. A major factor contributing to our ignorance of spirochetal biology is the lack of methods for genetic analysis of these organisms. In this study, we have developed a system for random transposon mutagenesis of Leptospira biflexa using a mariner transposon, Himar1. To demonstrate the validity of Himar1 in vivo transposon mutagenesis in L. biflexa, a screen of mutants for clones impaired in amino acid biosynthesis was first performed, enabling the identification of tryptophan and glutamate auxotrophs. To investigate iron transporters, 2,000 L. biflexa transposon mutants were screened onto media with and without hemin, thus allowing the identification of five hemin-requiring mutants, and the putative genes responsible for this phenotype were identified. Three mutants had distinct insertions in a gene encoding a protein which shares homology with the TonB-dependent receptor FecA, involved in ferric citrate transport. We also identified two mutants with a Himar1 insertion into a feoB-like gene, the product of which is required for ferrous iron uptake in many bacterial organisms. Interestingly, the growth inhibition exhibited by the fecA and feoB mutants was relieved by deferoxamine, suggesting the presence of a ferric hydroxamate transporter. These results confirm the importance of iron for the growth of Leptospira and its ability to use multiple iron sources.

Transgenic expression of RecA of the spirochetes Borrelia burgdorferi and Borrelia hermsii in Escherichia coli revealed differences in DNA repair and recombination phenotypes

After unsuccessful attempts to recover a viable RecA-deficient mutant of the Lyme borreliosis agent Borrelia burgdorferi, we characterized the functional activities of RecA of B. burgdorferi, as well as RecA of the relapsing fever spirochete Borrelia hermsii and the free-living spirochete Leptospira biflexa, in a recA mutant of Escherichia coli. As a control, E. coli RecA was expressed from the same plasmid vector. DNA damage repair activity was assessed after exposure of the transgenic cells to UV light or the radiomimetic chemicals methyl methanesulfonate and mitomycin C. Recombination activity in the cells was assessed by using an assay for homologous recombination between repeats in the chromosome and by measuring the ability of the cells to foster lytic growth by red gam mutant bacteriophage lambda. Overall, we found that transgenic cells with recA genes of B. burgdorferi, B. hermsii, and L. biflexa had approximately equivalent activities in promoting homologous recombination in the lacZ duplication assay, but cells with B. burgdorferi recA and, most notably, B. hermsii recA were significantly less capable than cells with L. biflexa recA or E. coli recA in responding to DNA damage or in facilitating plaque formation in the phage assay. The comparatively poor function of Borrelia recA in the latter set of assays may be the consequence of impaired coordination in the loading of the transgenic RecA by RecBCD and/or RecFOR in E. coli.

Functional properties of Borrelia burgdorferi recA

Functions of the Borrelia burgdorferi RecA protein were investigated in Escherichia coli recA null mutants. Complementation with B. burgdorferi recA increased survival of E. coli recA mutants by 3 orders of magnitude at a UV dose of 2,000 microJ/cm(2). The viability at this UV dose was about 10% that provided by the homologous recA gene. Expression of B. burgdorferi recA resulted in survival of E. coli at levels of mitomycin C that were lethal to noncomplemented hosts. B. burgdorferi RecA was as effective as E. coli RecA in mediating homologous recombination in E. coli. Furthermore, E. coli lambda phage lysogens complemented with B. burgdorferi recA produced phage even in the absence of UV irradiation. The level of phage induction was 55-fold higher than the level in cells complemented with the homologous recA gene, suggesting that B. burgdorferi RecA may possess an enhanced coprotease activity. This study indicates that B. burgdorferi RecA mediates the same functions in E. coli as the homologous E. coli protein mediates. However, the rapid loss of viability and the absence of induction in recA expression after UV irradiation in B. burgdorferi suggest that recA is not involved in the repair of UV-induced damage in B. burgdorferi. The primary role of RecA in B. burgdorferi is likely to be a role in some aspect of recombination.

Tomographic reconstruction of treponemal cytoplasmic filaments reveals novel bridging and anchoring components

An understanding of the involvement of bacterial cytoplasmic filaments in cell division requires the elucidation of the structural organization of those filamentous structures. Treponemal cytoplasmic filaments are composed of one protein, CfpA, and have been demonstrated to be involved in cell division. In this study, we used electron tomography to show that the filaments are part of a complex with a novel molecular organization that includes at least two distinct features decorating the filaments. One set of components appears to anchor the filaments to the cytoplasmic membrane. The other set of components appears to bridge the cytoplasmic filaments on the cytoplasmic side, and to be involved in the interfilament spacing within the cell. The filaments occupy between 3 and 18% of the inner surface of the cytoplasmic membrane. These results reveal a novel filamentous molecular organization of independent filaments linked by bridges and continuously anchored to the membrane.

Genetic evidence for the existence of two pathways for the biosynthesis of methionine in the Leptospira spp

There are two major pathways for methionine biosynthesis: the enterobacterial type transsulfuration pathway and the sulfhydrylation pathway as previously identified in the spirochete Leptospira meyeri. Sequence analysis of the L. meyeri metYX locus allows the identification of a third gene, called metW, which encodes a protein exhibiting similarities with homologs in many organisms belonging to the alpha-, beta-, and gamma-subdivisions of proteobacteria. The metW, metX and metY genes of L. meyeri were disrupted by a resistance cassette by homologous recombination. While the L. meyeri metX mutant shows methionine auxotrophy, the metY mutant (as well as the metW and metYmetW mutants) conserves methionine prototrophy, suggesting the presence of additional route(s) which may bypass the direct sulfhydrylation pathway. In addition, a L. interrogans gene, called metZ, was found to complement an Escherichia coli metB mutant, further suggesting that the transsulfuration pathway is also present in Leptospira spp.

Leptospira spp. possess a complete haem biosynthetic pathway and are able to use exogenous haem sources

Unlike the spirochetes Borrelia burgdorferi and Treponema pallidum, Leptospira spp. contain genes encoding the enzymes for most biosynthetic pathways. In this study, we describe the first haem biosynthetic pathway genes in the order Spirochaetales. Sequence analysis of the L. interrogans genome shows that all haem biosynthetic genes (hemA, heml, hemB, hemC, hemE, hemN, hemY and hemH) are clustered in a 15 kb region of the CII secondary chromosome. Although no hemD homologue (encoding uroporphyrinogen III synthase) was found in the genome, the L. interrogans hemC gene (encoding porphobilinogen deaminase) was able to restore uroporphyrinogen III synthase activity in an Escherichia coli Delta hemD mutant, suggesting that the L. interrogans hemC gene encodes a bifunctional enzyme. Similarly, we show that the L. interrogans hemH gene (encoding ferrochelatase, the terminal enzyme of the haem biosynthetic pathway) is able to complement a ferrochelatase-defective E. coli Delta hemH mutant. Further investigation of ferrochelatases was undertaken in both saprophytic and pathogenic species of Leptospira. Ferrochelatase activity of 2.3 +/- 0.1 nmol h-1 mg-1 (in comparison with 0.25 +/- 0.02 nmol h-1 mg-1 in E. coli) was found in membrane fractions of pathogenic and saprophytic species, suggesting that ferrochelatase is a membrane-associated protein. Leptospira biflexa allelic exchange mutants containing an inactivated hemH gene were recovered only when exogenous haemin was present. The results indicate that haem is an essential growth factor for Leptospira, and that these spirochetes are capable of both de novo synthesis or uptake of haem. This may have implications in a better understanding of the pathogenesis of Leptospira.

Temperature-dependent hypermutational phenotype in recA mutants of Thermus thermophilus HB27

The recA gene from Thermus thermophilus HB27 was cloned and engineered to obtain insertion (recA::kat) and deletion (deltarecA) derivatives. Transcription of recA in this extreme thermophile was induced by mitomycin C, leading to the synthesis of a monocistronic mRNA. This DNA damage-mediated induction was dependent on the integrity of recA. In addition to UV sensitivity, the recA mutants of T. thermophilus showed severe pleiotropic defects, ranging from irregular nucleoid condensation and segregation to a dramatic reduction in viability during culture. An increase in the frequency of both carotenoidless and auxotrophic mutants within surviving cells of the deltarecA strain indicated a high mutation rate. As RecA is not required for plasmid transformation, we have used the alpha-lacZ gene fragment and the ampicillin resistance gene from Escherichia coli as passenger reporters to confirm such high mutation rates. Our data support the idea that the absence of RecA results in a hypermutational phenotype in T. thermophilus. Furthermore, a direct relationship is deduced between the growth temperature and mutation rate, which finally has a deleterious effect on cell survival in the absence of RecA.

Construction and complementation of the first auxotrophic mutant in the spirochaete Leptospira meyeri

In bacteria, the first reaction of the tryptophan biosynthetic pathway involves the conversion of chorismate and glutamine to anthranilate by the action of anthranilate synthase, which is composed of the alpha (trpE gene product) and beta (trpG gene product) subunits. In this study, the tryptophan biosynthetic gene trpE of the spirochaete Leptospira meyeri was interrupted by a kanamycin-resistance cassette by homologous recombination. The trpE double cross-over mutant was not able to grow on solid or in liquid EMJH medium. In contrast, the trpE mutant showed a wild-type phenotype when tryptophan or anthranilate was added to the media, therefore showing that disruption of the L. meyeri trpE gene resulted in tryptophan auxotrophy. The authors have also characterized a second selectable marker that allows the construction of a spectinomycin-resistant L. meyeri-E. coli shuttle vector and the functional complementation of the L. meyeri trpE mutant.