High-frequency conjugal plasmid transfer from gram-negative Escherichia coli to various gram-positive coryneform bacteria

Cloning of the pyruvate kinase gene (pyk) of Corynebacterium glutamicum and site-specific inactivation of pyk in a lysine-producing Corynebacterium lactofermentum strain

The pyruvate kinase gene pyk from Corynebacterium glutamicum was cloned by applying a combination of PCR, site-specific mutagenesis, and complementation. A 126-bp DNA fragment central to the C. glutamicum pyk gene was amplified from genomic DNA by PCR with degenerate oligonucleotides as primers. The cloned DNA fragment was used to inactivate the pyk gene in C. glutamicum by marker rescue mutagenesis via homologous recombination. The C. glutamicum pyk mutant obtained was unable to grow on minimal medium containing ribose as the sole carbon source. Complementation of this phenotype by a gene library resulted in the isolation of a 2.8-kb PstI-BamHI genomic DNA fragment harboring the C. glutamicum pyk gene. Multiple copies of plasmid-borne pyk caused a 20-fold increase of pyruvate kinase activity in C. glutamicum cell extracts. By using large internal fragments of the cloned C. glutamicum gene, pyk mutant derivatives of the lysine production strain Corynebacterium lactofermentum 21799 were generated by marker rescue mutagenesis. As determined in shake flask fermentations, lysine production in pyk mutants was 40% lower than that in the pyk+ parent strain, indicating that pyruvate kinase is essential for high-level lysine production. This finding questions an earlier hypothesis postulating that redirection of carbon flow at the phosphoenol pyruvate branch point of glycolysis through elimination of pyruvate kinase activity results in an increase of lysine production in C. glutamicum and its close relatives.

Increased fertility of Corynebacterium glutamicum recipients in intergeneric matings with Escherichia coli after stress exposure

Corynebacterial recipient cells exposed to heat, organic solvents, pH shifts, or detergents show an increased fertility in subsequent interspecific matings with Escherichia coli. This effect is independent of de novo protein biosynthesis and seems to be due to a direct inactivation of a restriction system active against foreign DNA that enters the cell by IncP-mediated conjugation.

A general method for identification of polyhydroxyalkanoic acid synthase genes from pseudomonads belonging to the rRNA homology group I

Using a 30-mer oligonucleotide probe highly specific for polyhydroxyalkanoic acid (PHA) synthase genes, the respective genes of Pseudomonas citronellolis, P. mendocina, Pseudomonas sp. DSM 1650 and Pseudomonas sp. GP4BH1 were cloned from genomic libraries in the cosmid pHC79. A 19.5-kbp and a 22.0-kbp EcoRI restriction fragment of P. citronellolis or Pseudomonas sp. DSM 1650, respectively, conferred the ability to accumulate PHA of medium-chain-length 3-hydoxyalkanoic acids (HAMCL) from octanoate as well as from gluconate to the PHA-negative mutant P. putida GPp104. An 11.0-kbp EcoRI fragment was cloned from P. mendocina, which restored in GPp104 the ability to synthesize PHA from octanoate but not from gluconate. From Pseudomonas sp. GP4BH1 three different genomic fragments encoding PHA synthases were cloned. This indicated that strain GP4BH1 possesses three different functionally active PHA synthases. Two of these fragments (6.4 kbp and 3.8 kbp) encoded for a PHA synthase, preferentially incorporating hydroxyalkanoic acids of short chain length (HASCL), and the synthases were expressed in either GPp104 and Alcaligenes eutrophus H16-PHB-4, respectively. The PHA synthase encoded by the third fragment (6.5 kbp) led to the incorporation of HAMCL and was expressed in GPp104 but not in PHB-4.

Molecular aspects of lysine, threonine, and isoleucine biosynthesis in Corynebacterium glutamicum

The Gram-positive bacterium Corynebacterium glutamicum is used for the industrial production of amino acids, e.g. of L-glutamate and L-lysine. In the last ten years genetic engineering methods were developed for C. glutamicum and consequently, recombinant DNA technology was employed to study the biosynthetic pathways and to improve the amino acid productivity by manipulation of enzymatic, transport and regulatory functions of this bacterium. The present review summarizes the current knowledge on the synthesis and over-production of the aspartate derived amino acids L-lysine, L-threonine and L-isoleucine in C. glutamicum. A special feature of C. glutamicum is its ability to convert the lysine intermediate piperideine2,6-dicarboxylate to diaminopimelate by two different routes, i.e. by reactions involving succinylated intermediates or by the single reaction of diaminopimelate dehydrogenase. The flux distribution over the two pathways is regulated by the ammonium availability. The overall carbon flux from aspartate to lysine, however, is governed by feedback-control of the aspartate kinase and by the level of dihydrodipicolinate synthase. Consequently, expression of lysCFBR encoding a deregulated aspartate kinase and/or the overexpression of dapA encoding dihydrodipicolinate synthase led to overproduction of lysine. As a further specific feature C. glutamicum possesses a specific lysine export carrier which shows high activity in lysine overproducing mutants. Threonine biosynthesis is in addition to control by the aspartate kinase tightly regulated at the level of homoserine dehydrogenase which is subject to feedback-inhibition and to repression. C. glutamicum strains possessing a deregulated aspartate kinase and a deregulated homoserine dehydrogenase produce lysine and threonine. Amplification of deregulated homoserine dehydrogenase in such strains led to an almost complete redirection of the carbon flux to threonine. For a further flux from threonine to isoleucine the allosteric control of threonine dehydratase and of the acetohydroxy acid synthase are important. The expression of the genes encoding the latter enzyme is additionally regulated at the transcriptional level. By addition of 2-oxobutyrate as precursor and by bypassing the expression control of the acetohydroxy acid synthase genes high isoleucine overproduction can be obtained.

Leucine synthesis in Corynebacterium glutamicum: enzyme activities, structure of leuA, and effect of leuA inactivation on lysine synthesis

Enzymes and genes of the isopropylmalate pathway leading to leucine in Corynebacterium glutamicum were studied, and assays were performed to unravel their connection to lysine oversynthesis. The first enzyme of the pathway is inhibited by leucine (Ki = 0.4 mM), and all three enzyme activities of the isopropylmalate pathway are reduced upon addition of this amino acid to the growth medium. Three different DNA fragments were cloned, each resulting in an oversynthesis of one of the three enzymes. The leuA complementing fragment encoding the isopropylmalate synthase was sequenced. The leuA gene is 1,848 bp in size, encoding a polypeptide with an M(r) of 68,187. Upstream of leuA there is extensive hyphenated dyad symmetry and a putative leader peptide, which are features characteristic of attenuation control. In addition to leuA, the sequenced fragment contains an open reading frame with high coding probability whose disruption did not result in a detectable phenotype. Furthermore, the sequence revealed that this open reading frame separates leuA from lysC, which encodes the aspartate kinase initiating the synthesis of all amino acids of the aspartate family. The leuA gene was inactivated in three lysine-secreting strains by insertional mutagenesis. Fermentations were performed, and a roughly 50% higher lysine yield was obtained when appropriate leucine concentrations limiting for growth of the constructed strains were used.

Stable Expression of hom-1-thrB in Corynebacterium glutamicum and Its Effect on the Carbon Flux to Threonine and Related Amino Acids

The hom-1-thrB operon encodes homoserine dehydrogenase resistant to feedback inhibition by L-threonine and homoserine kinase. Stable expression of this operon has not yet been attained in different Corynebacterium glutamicum strains. We studied the use of chromosomal integration and of a low-copy-number vector for moderate expression of the hom-1-thrB operon to enable an analysis of the physiological consequences of its expression in C. glutamicum. Strains carrying one, two, or three copies of hom-1-thrB were obtained. They showed proportionally increased enzyme activity of feedback-resistant homoserine dehydrogenase and of homoserine kinase. This phenotype was stably maintained in all recombinants for more than 70 generations. In a lysine-producing C. glutamicum strain which does not produce any threonine, expression of one copy of hom-1-thrB resulted in the secretion of 39 mM threonine. Additional copies resulted in a higher, although not proportional, accumulation of threonine (up to 69 mM). This indicates further limitations of threonine production. As the copy number of hom-1-thrB increased, increasing amounts of homoserine (up to 23 mM) and isoleucine (up to 34 mM) were secreted. Determination of the cytosolic concentration of the respective amino acids revealed an increase of intracellular threonine from 9 to 100 mM and of intracellular homoserine from 4 to 74 mM as the copy number of hom-1-thrB increased. These results suggest that threonine production with C. glutamicum is limited by the efflux system for this amino acid. Furthermore, the results show the successful use of moderate and stable hom-1-thrB expression for directing the carbon flux from aspartate to threonine.

A gene encoding arginyl-tRNA synthetase is located in the upstream region of the lysA gene in Brevibacterium lactofermentum: regulation of argS-lysA cluster expression by arginine

The Brevibacterium lactofermentum argS gene, which encodes an arginyl-tRNA synthetase, was identified in the upstream region of the lysA gene. The cloned gene was sequenced; it encodes a 550-amino-acid protein with an M(r) of 59,797. The deduced amino acid sequence showed 28% identical and 49% similar residues when compared with the sequence of the Escherichia coli arginyl-tRNA synthetase. The B. lactofermentum enzyme showed the highly conserved motifs of class I aminoacyl-tRNA synthetases. Expression of the argS gene in B. lactofermentum and E. coli resulted in an increase in aminoacyl-tRNA synthetase activity, correlated with the presence in sodium dodecyl sulfate-polyacrylamide gels of a clear protein band that corresponds to this enzyme. One single transcript of about 3,000 nucleotides and corresponding to the B. lactofermentum argS-lysA operon was identified. The transcription of these genes is repressed by lysine and induced by arginine, showing an interesting pattern of biosynthetic interlock between the pathways of both amino acids in corynebacteria.

Isoleucine synthesis in Corynebacterium glutamicum: molecular analysis of the ilvB-ilvN-ilvC operon

Acetohydroxy acid synthase (AHAS) and isomeroreductase (IR) catalyze subsequent reactions in the flux of metabolites towards isoleucine, valine, leucine, and pantothenate. A 4,705-bp DNA fragment from Corynebacterium glutamicum known to code for AHAS and IR was sequenced and analyzed by Northern (RNA blot) analysis. As in other bacteria, the AHAS of this gram-positive organism is encoded by two genes, ilvB and ilvN. Gene disruption verified that these genes encode the single AHAS activity in C. glutamicum. The start of ilvB was determined by amino-terminal sequencing of a fusion peptide. By Northern analysis of the ilvBNC cluster, three in vivo transcripts of 3.9, 2.3, and 1.1 kb were identified, corresponding to ilvBNC, ilvNC, and ilvC messages, respectively. The ilvC transcript (encoding IR) was by far the most abundant one. With a clone from which the ilvB upstream regions had been deleted, only the ilvNC and ilvC transcripts were synthesized, and with a clone from which the ilvN upstream regions had been deleted, only the smallest ilvC transcript was formed. It is therefore concluded that in the ilv operon of C. glutamicum, three promoters are active. The amounts of the ilvBNC and ilvNC transcripts increased in response to the addition of alpha-ketobutyrate to the growth medium. This was correlated to an increase in specific AHAS activity, whereas IR activity was not increased because of the relatively large amount of the ilvC transcript present under all conditions assayed. Therefore, the steady-state level of the ilvBNC and ilvNC messages contributes significantly to the total activity of the single AHAS. The ilvC transcript of this operon, however, is regulated independently and present in a large excess, which is in accord with the constant IR activities determined.

Glutamate Dehydrogenase Is Not Essential for Glutamate Formation by Corynebacterium glutamicum

Two Corynebacterium glutamicum strains, one being glutamate dehydrogenase (GDH) negative and the other possessing 11-fold-higher specific GDH activity than the parental wild type, were constructed and used to analyze the role of GDH in C. glutamicum . The results indicate (i) that GDH is dispensable for glutamate synthesis required for growth and (ii) that although a high level of GDH increases the intracellular glutamate pool, the level of GDH has no influence on glutamate secretion.

Strains of Corynebacterium glutamicum with Different Lysine Productivities May Have Different Lysine Excretion Systems

The lysine excretion systems of three different lysine-producing strains of Corynebacterium glutamicum were characterized in intact cells. Two strains (DG 52-5 and MH 20-22B) are lysine producers of different efficiency. They were bred by classical mutagenesis and have a feedback-resistant aspartate kinase. The third strain (KK 25) was constructed from the wild type by introducing the feedback-resistant aspartate kinase gene of strain MH 20-22B into its genome. The three strains were shown to possess different excretion systems. Export in strain KK 25 is much slower than in the two mutants. The differences between the two lysine-producing strains are more subtle. K m and V max are similar, but pH dependence and membrane potential dependence reveal differences in the intrinsic properties of the carrier system.

Determination of the mechanism of retrotransfer by mechanistic mathematical modeling

Two mathematical models to elucidate the mechanism of retromobilization (or retrotransfer), that is, the ability of conjugative plasmids to mobilize genes into the cell containing the conjugative plasmid, were developed. This study deals with retromobilization of nonconjugative plasmids (Tra-Mob+). Plasmid transfer was modeled by two mass action models. The first is based on the hypothesis that retromobilization of the Tra-Mob+ vector occurs in one step, by means of the pilus formed by the Tra+ plasmid in the original host. In the second model, retromobilization is considered to be a two-step process involving two transfer events. The first step involves the transfer of the Tra+ plasmid from the recipient cell to the donor of the nonconjugative vector, and during the second encounter the nonconjugative vector is mobilized toward the recipient. Since the relationships between the number of transconjugants and the number of recipients for the two models are different, filter matings were performed for short time periods with different initial densities of the recipient population. Comparison of the numbers of transconjugants with the results of the mathematical equations confirmed the hypothesis that retromobilization is a one-step conjugation process.

Expression of the Bacillus subtilis sacB gene leads to sucrose sensitivity in the gram-positive bacterium Corynebacterium glutamicum but not in Streptomyces lividans

The expression of the structural gene (sacB) encoding Bacillus subtilis levansucrase in two gram-positive soil bacteria, Corynebacterium glutamicum ATCC 13032 and Streptomyces lividans 1326, was investigated. sacB expression in the presence of sucrose is lethal to C. glutamicum but not to S. lividans. While S. lividans secretes levansucrase into the medium, we could show that the enzyme is retained by C. glutamicum cells. Our results imply that the sacB gene can be used as a positive selection system in coryneform bacteria.

Transformation of the gram-positive bacterium Clavibacter xyli subsp. cynodontis by electroporation with plasmids from the IncP incompatibility group

We report the transformation of a gram-positive bacterium, Clavibacter xyli subsp. cynodontis, with several plasmids in the IncP incompatibility group from gram-negative bacteria. Our results suggest that IncP plasmids may be transferable to other gram-positive organisms. After optimizing electroporation parameters, we obtained a maximum of 2 x 10(5) transformants per microgram of DNA. The availability of a transformation system for this bacteria will facilitate its use in indirectly expressing beneficial traits in plants.

The Corynebacterium glutamicum aecD gene encodes a C-S lyase with alpha, beta-elimination activity that degrades aminoethylcysteine

S-(beta-Aminoethyl)-cysteine (AEC) resistance was achieved in Corynebacterium glutamicum by cloning a chromosomal 1.5-kb EcoRV-BglII DNA fragment on a multicopy plasmid. DNA sequence analysis of the 1.5-kb DNA fragment revealed an open reading frame (ORF326) which represents the AEC resistance gene, designated aecD. The aecD gene directs the synthesis of a 36-kDa protein which was visualized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The aecD gene is a nonessential gene and mediates AEC resistance only in an amplified state. C. glutamicum strains harboring an amplified aecD gene can utilize AEC as an alternative nitrogen source, indicating that the AEC resistance mechanism is due to AEC degradation. Since the AEC degradation products analyzed by high-pressure liquid chromatography were found to be pyruvate and aminoethanethiol (cysteamine), it was concluded that the aecD gene encodes a C-S lyase with alpha, beta-elimination activity. Besides AEC, the C-S lyase was also able to use cysteine, cystine, and cystathionine as substrates.

Molecular analysis of the Corynebacterium glutamicum lysl gene involved in lysine uptake

Two Corynebacterium glutamicum mutants defective in lysine uptake were identified by analysing mutants resistant to S-(2-aminoethyl)-cysteine (AEC). A 5.6 kb genomic DNA fragment restoring AEC sensitivity and lysine uptake was isolated. A 4.2 kb subfragment was sequenced and three open reading frames were identified. Subcloning and gene disruption experiments showed that only the first open reading frame, termed lysl, is involved in lysine uptake. Lysl consists of 501 amino acids with a Mr of 53600. The hydrophobicity profile suggests that the lysl gene product is an integral membrane protein with 13 transmembrane segments. The amino acid sequence of lysl displays strong homology to that of the arcD gene product of Pseudomonas aeruginosa, which is proposed to act as an arginine-ornithine antiporter. Investigation of the influence of the lysl gene on lysine secretion suggests the existence of a separate lysine efflux system in C. glutamicum.

Genetic exchange between kingdoms

Bacterial conjugation with two evolutionarily divergent yeasts has been observed in the laboratory. Whether such trans-kingdom conjugation events, other than the well known Agrobacterium-plant cell interaction, actually occur in nature is not known. However, a few putative events have recently been uncovered by gene (or protein) sequence analysis, suggesting that horizontal gene transfer between phylogenetic kingdoms may be a real phenomenon.

Transfer of plasmid RSF1010 by conjugation from Escherichia coli to Streptomyces lividans and Mycobacterium smegmatis

The plasmid RSF1010 belongs to a class of plasmids (IncQ) that replicate in a range of bacterial hosts. Although non-self-transmissible, it can be mobilized at high frequency between different gram-negative bacterial species if transfer functions are supplied in trans. We report the transfer of RSF1010 by conjugation from Escherichia coli to the gram-positive actinomycetes Streptomyces lividans and Mycobacterium smegmatis. In its new hosts, the plasmid was stable with respect to structure and inheritance and conferred high-level resistance to streptomycin and sulfonamide. This is the first reported case of conjugative transfer of a naturally occurring plasmid between gram-negative and gram-positive bacteria.

'Integron'-bearing vectors: a method suitable for stable chromosomal integration in highly restrictive corynebacteria

A pBR322-derived plasmid (pCGL107) that carries the Corynebacterium melassecola ATCC17965 analogue of Escherichia coli gdhA gene (encoding glutamate dehydrogenase), was introduced into the related strain, Brevibacterium lactofermentum CGL2002, by electroporation and integrated into its chromosome by homologous recombination. However, pCGL107 cannot integrate into C. melassecola, since the host restriction prevents successful electroporation by E. coli-modified DNA. Nevertheless, B. lactofermentum-modified replicative plasmid DNA can be transformed by electroporation into C. melassecola; thus pCGL519-2, a shuttle plasmid that carries the C. melassecola analogue of E. coli gltA (encoding citrate synthase), was extracted from the former host and electroporated into the latter. Rare restriction sites conveniently placed in pCGL519-2 were used to recover a replicon-less cartridge called 'integron', that contains a selectable marker and gtlA within a single fragment. Integron prepared from pCGL519-2 DNA which had been extracted from C. melassecola, and thus, was capable of eluding the C. melassecola restriction barrier(s), was successfully electroporated into this host. The molecular analysis of the resulting transformants suggests that they result from the integration of a single circular integron molecule by homologous recombination between the gltA regions of the host genome and the integron. These transformants were stable for 30 generations in the absence of selection.

Characterization of pGA1, a new plasmid from Corynebacterium glutamicum LP-6

A new plasmid, pGA1, has been isolated from Corynebacterium glutamicum LP-6, and its detailed restriction map has been prepared. The 4.9-kb plasmid has a G + C content of 57%. It replicates in C. glutamicum ATCC13032 and is compatible with the three other plasmids, pCC1, pBL1 and pHM1519, commonly used for vector construction for amino acid-producing corynebacteria. Fusions of pGA1 with different Escherichia coli replicons (transferred from E. coli to Corynebacterium via transformation of spheroplasts or by filter mating experiments with intact cells) are shown to be suitable as shuttle plasmids; some of them are highly stable in C. glutamicum, even when propagated without any selection pressure.

Cloning vectors and antibiotic-resistance markers for Brevibacterium sp. R312

Replication of several cryptic plasmids from coryneform strains was investigated in Brevibacterium sp. R312. Only the Corynebacterium glutamicum pSR1 replicon was found to be suitable for establishing a host-vector system. Two pSR1 derivatives, pRPCG200 and pHYCG1, were used as cloning vectors. They carry a neomycin-resistance-encoding and a tetracycline-resistance-encoding gene, respectively.

Manipulation of Corynebacterium glutamicum by gene disruption and replacement

We have developed a system for the genetic manipulation of the amino acid-producing Corynebacterium glutamicum. Gene disruption and replacement were achieved by introducing, via conjugation, Escherichia coli vector plasmids carrying manipulated C. glutamicum DNA fragments. We obtained stable mutants in which the chromosomal lysA gene, encoding meso-diaminopimelate decarboxylase, was interrupted by a chloramphenicol resistance cartridge, or in which an essential internal part of the lysA gene was deleted. The deletion mutants retain neither antibiotic resistance markers nor vector sequences. This strategy is generally applicable to the construction of industrial strains to be used in fermentation processes.

The susceptibility of conjugative resistance transfer in gram-negative bacteria to physicochemical and biochemical agents

Over thirty years of studies have established that conjugative transfer of plasmid-encoded resistance to drugs and heavy metals can take place at high frequency between various organisms under laboratory conditions. The detected transfer frequencies in soil, in aquatic environments, and in the urogenital and respiratory tracts of healthy animals and man have generally been low. However, the conversion of bacteria from susceptible to resistant to antibiotics has been observed often during antimicrobial therapy. This has formed a challenge for the antibacterial treatment of pathogenic bacteria and called for the evaluation of the extent of conjugative transfer in various environments. Several biochemical and physicochemical factors inhibit conjugation, show preferential toxicity against plasmid-bearing cells, or stimulate plasmid curing. These factors include various agents such as detergents, anesthetics, mutagens and antibiotics which affect membrane potential, membrane permeability, protein synthesis and the processing of DNA. The application of the data on these agents, summarized in this review, might be helpful in preventing drug multi-resistance from spreading. Also these data might be valuable in studies which use conjugation as a tool or which treat the molecular mechanisms involved in conjugation.

A functionally split pathway for lysine synthesis in Corynebacterium glutamicium

Three different pathways of D,L-diaminopimelate and L-lysine synthesis are known in procaryotes. Determinations of the corresponding enzyme activities in Escherichia coli, Bacillus subtilis, and Bacillus sphaericus verified the fact that in each of these bacteria only one of the possible pathways operates. However, in Corynebacterium glutamicum activities are present which allow in principle the use of the dehydrogenase variant and succinylase variant of lysine synthesis together. Applying gene-directed mutagenesis, various C. glutamicum strains were constructed with interrupted ddh gene. These mutants have an inactive dehydrogenase pathway but are still prototrophic, which is proof that the succinylase pathway of D,L-diaminopimelate synthesis can be utilized. In strains with an increased flow of precursors to D,L-diaminopimelate, however, the inactivation of the dehydrogenase pathway resulted in a reduced formation of lysine, with concomitant accumulation of N-succinyl-diaminopimelate in the cytosol up to a concentration of 25 mM. These data show (i) that both pathways can operate in C. glutamicum for D,L-diaminopimelate and L-lysine synthesis, (ii) that the dehydrogenase pathway is not essential, and (iii) that the dehydrogenase pathway is a prerequisite for handling an increased flow of metabolites to D,L-diaminopimelate.

Shuttle vectors containing a multiple cloning site and a lacZ alpha gene for conjugal transfer of DNA from Escherichia coli to gram-positive bacteria

The mobilizable shuttle cloning vectors, pAT18 and pAT19, are composed of: (i) the replication origins of pUC and of the broad-host-range enterococcal plasmid pAM beta 1; (ii) an erythromycin-resistance-encoding gene expressed in Gram- and Gram+ bacteria; (iii) the transfer origin of the IncP plasmid RK2; and (iv) the multiple cloning site and the lacZ alpha reporter gene of pUC18 (pAT18) and pUC19 (pAT19). These 6.6-kb plasmids contain ten unique cloning sites that allow screening of derivatives containing DNA inserts by alpha-complementation in Escherichia coli carrying the lacZ delta M15 deletion, and can be efficiently mobilized by self-transferable IncP plasmids co-resident in the E. coli donors. Plasmids pAT18, pAT19 and recombinant derivatives have been successfully transferred by conjugation from E. coli to Bacillus subtilis, Bacillus thuringiensis, Listeria monocytogenes, Enterococcus faecalis, Lactococcus lactis, and Staphylococcus aureus at frequencies ranging from 10(-6) to 10(-9). The presence of a restriction system in the recipient dramatically affects (by three orders of magnitude) the efficiency of conjugal transfer of these vectors from E. coli to Gram+ bacteria.

Studies on the utilization of lactose by Corynebacterium glutamicum, bearing the lactose operon of Escherichia coli

The entire Escherichia coli lactose operon was inserted into an E. coli/Corynebacterium glutamicum shuttle vector and introduced into the gram-positive host organism C. glutamicum R 163. Recombinant C. glutamicum strains carrying the lac genes downstream of an efficient promoter displayed rapid growth with lactose as the sole source of carbon. Two prerequisites were necessary to obtain good growth of C. glutamicum R 163 on lactose: presence of the lacY gene in addition to lacZ and an appropriate promoter for efficient transcription in C. glutamicum. The galactose moiety of lactose was not utilized but accumulated in the culture broth. C. glutamicum strains carrying only the lacZ (beta-galactosidase) gene but not lacY (lactose permease) were not able to grow in lactose minimal medium.

Nucleotide sequence and functions of the oriT operon in IncI1 plasmid R64

Two transfer genes of IncI1 plasmid R64, tentatively designated nikA and nikB, were cloned and sequenced. They are located adjacent to the origin of transfer (oriT) and appear to be organized into an operon, which we call the oriT operon. On the basis of the DNA sequence, nikA and nikB were concluded to encode proteins with 110 and 899 amino acid residues, respectively. Complementation analysis indicated that these two genes are indispensable for the transfer of R64 but are not required for the mobilization of ColE1. By the maxicell procedure, the product of nikA was found to be a 15-kDa protein. On treating a cleared lysate prepared from cells harboring a plasmid containing oriT, nikA, and nikB with sodium dodecyl sulfate or proteinase K, superhelical plasmid DNA in the cleared lysate was converted to an open circular form (relaxation). Relaxation of plasmid DNA was found to require the oriT sequence in cis and the nikA and nikB sequences in trans. It would thus follow that the products of nikA and nikB genes form a relaxation complex with plasmid DNA at the oriT site.

Biolistic transformation of a procaryote, Bacillus megaterium

We present a simple and rapid method for introducing exogenous DNA into a bacterium, Bacillus megaterium, utilizing the recently developed biolistic process. A suspension of B. megaterium was spread onto the surface of nonselective medium. Plasmid pUB110 DNA, which contains a gene that confers kanamycin resistance, was precipitated onto tungsten particles. Using a biolistic propulsion system, the coated particles were accelerated at high velocities into the B. megaterium recipient cells. Selection was done by use of an agar overlay containing 50 micrograms of kanamycin per ml. Antibiotic-resistant transformants were recovered from the medium interface after 72 h of incubation, and the recipient strain was shown to contain the delivered plasmid by agarose gel electrophoresis of isolated plasmid DNA. All strains of B. megaterium tested were successfully transformed by this method, although transformation efficiency varied among strains. Physical variables of the biolistic process and biological variables associated with the target cells were optimized, yielding greater than 10(4) transformants per treated plate. This is the first report of the biolistic transformation of a procaryote.

Isolation and characterization of insertion sequence elements from gram-negative bacteria by using new broad-host-range, positive selection vectors

On the basis of an RSF1010-derived broad-host-range vector, three different systems which enable positive detection and isolation of insertion sequence (IS) elements from gram-negative bacteria were constructed. Vectors pSUP104-pheS, pSUP104-rpsL, and pSUP104-sac were used successfully in a number of Rhizobium strains and in Xanthomonas campestris. More than 20 different IS elements were isolated and characterized. The 16 IS elements from Rhizobium meliloti were further used to characterize various R. meliloti strains by hybridization. The resulting hybridization patterns were different for every strain and gave a clear and definite IS fingerprint of each strain. These IS fingerprints can be used to identify and characterize R. meliloti strains rapidly and unequivocally, as they proved to be relatively stable. Some of the IS elements were found to be identical when the IS fingerprints from a given strain were compared. This method of IS fingerprinting can also establish whether IS elements are the same, related, or different.

Nucleotide sequence and organization of genes flanking the transfer origin of promiscuous plasmid RP4

The nucleotide sequence of the relaxase operon and the leader operon which are part of the Tra1 region of the promiscuous plasmid RP4 was determined. These two polycistronic operons are transcribed divergently from an intergenic region of about 360 bp containing the transfer origin and six close-packed genes. A seventh gene completely overlaps another one in a different reading frame. Conjugative DNA transfer proceeds unidirectionally from oriT with the leader operon heading the DNA to be transferred. The traI gene of the relaxase operon includes within its 3' terminal region a promoter controlling the 7.2-kb polycistronic primase operon. Comparative sequence analysis of the closely related IncP plasmid R751 revealed a similarity of 74% at the nucleotide sequence level, indicating that RP4 and R751 have evolved from a common ancestor. The gene organization of relaxase- and leader operons is conserved among the two IncP plasmids. The transfer origins and the genes traJ and traK exhibit greater sequence divergence than the other genes of the corresponding operons. This is conceivable, because traJ and traK are specificity determinants, the products of which can only recognize homologous oriT sequences. Surprisingly, the organization of the IncP relaxase operons resembles that of the virD operon of Agrobacterium tumefaciens plasmid pTiA6 that mediates DNA transfer to plant cells by a process analogous to bacterial conjugation. Furthermore, the IncP TraG proteins and the product of the virD4 gene share extended amino acid sequence similarity, suggesting a functional relationship.

Natural transfer of conjugative transposon Tn916 between gram-positive and gram-negative bacteria

The conjugative streptococcal transposon Tn916 was found to transfer naturally between a variety of gram-positive and gram-negative eubacteria. Enterococcus faecalis hosting the transposon could serve as a donor for Alcaligenes eutrophus, Citrobacter freundii, and Escherichia coli at frequencies of 10(-6) to 10(-8). No transfer was observed with several phototrophic species. Mating of an E. coli strain carrying Tn916 yielded transconjugants with Bacillus subtilis, Clostridium acetobutylicum, Enterococcus faecalis, and Streptococcus lactis subsp. diacetylactis at frequencies of 10(-4) to 10(-6). Acetobacterium woodii was the only gram-positive organism tested that did not accept the transposon from a gram-negative donor. The results prove the ability of conjugative transposable elements such as Tn916 for natural cross-species gene transfer, thus potentially contributing to bacterial evolution.

Transformation of the phytopathogenic bacterium Clavibacter michiganense subsp. michiganense by electroporation and development of a cloning vector

We constructed a cloning vector for use in the plant pathogenic bacterium Clavibacter michiganense subsp. michiganense. The vector pDM100 consists of a 3.2-kb restriction fragment of the Clavibacter plasmid pCM1 joined to a pBR325 derivative carrying the neomycin phosphotransferase of transposon Tn5 and the gentamicin acetyltransferase of Tn1696. Both antibiotic resistance genes are efficiently expressed in C. michiganense subsp. michiganense. Although polyethylene glycol-mediated transfection of spheroplasts with the DNA of the C. michiganense subsp. michiganense-specific bacteriophage CMP1 yielded about 3 x 10(3) transfectants per microgram of DNA, in transformations with plasmid DNA only a very few transformants were obtained. However, the transformation efficiency could be improved by electroporation of intact cells, giving about 2 x 10(3) transformants per microgram of plasmid DNA. Since a transformation procedure and a cloning vector are now available, pathogenicity in C. michiganense subsp. michiganense can now be analyzed genetically.

Organization of the bacterial chromosome

Recent progress in studies on the bacterial chromosome is summarized. Although the greatest amount of information comes from studies on Escherichia coli, reports on studies of many other bacteria are also included. A compilation of the sizes of chromosomal DNAs as determined by pulsed-field electrophoresis is given, as well as a discussion of factors that affect gene dosage, including redundancy of chromosomes on the one hand and inactivation of chromosomes on the other hand. The distinction between a large plasmid and a second chromosome is discussed. Recent information on repeated sequences and chromosomal rearrangements is presented. The growing understanding of limitations on the rearrangements that can be tolerated by bacteria and those that cannot is summarized, and the sensitive region flanking the terminator loci is described. Sources and types of genetic variation in bacteria are listed, from simple single nucleotide mutations to intragenic and intergenic recombinations. A model depicting the dynamics of the evolution and genetic activity of the bacterial chromosome is described which entails acquisition by recombination of clonal segments within the chromosome. The model is consistent with the existence of only a few genetic types of E. coli worldwide. Finally, there is a summary of recent reports on lateral genetic exchange across great taxonomic distances, yet another source of genetic variation and innovation.