Cloning, expression, and chromosomal stabilization of the Propionibacterium shermanii proline iminopeptidase gene (pip) for food-grade application in Lactococcus lactis

Proline iminopeptidase produced by Propionibacterium shermanii plays an essential role in the flavor development of Swiss-type cheeses. The enzyme (Pip) was purified and characterized, and the gene (pip) was cloned and expressed in Escherichia coli and Lactococcus lactis, the latter species being an extensively studied, primary cheese starter culture that is less fastidious in its growth condition requirements than P. shermanii. The levels of expression of the pip gene could be enhanced with a factor 3 to 5 by using a strong constitutive promoter in L. lactis or the inducible tac promoter in E. coli. Stable replication of the rolling-circle replicating (rcr) plasmid, used to express pip in L. lactis, could only be obtained by providing the repA gene in trans. Upon the integration of pip, clear gene dosage effects were observed and stable multicopy integrants could be maintained upon growth under the selective pressure of sucrose. The multicopy integrants demonstrated a high degree of stability in the presence of glucose. This study examines the possibilities to overexpress genes that play an important role in food fermentation processes and shows a variety of options to obtain stable food-grade expression of such genes in L. lactis.

Autolysis of Lactococcus lactis is influenced by proteolysis

The autolysin AcmA of Lactococcus lactis was shown to be degraded by the extracellular lactococcal proteinase PrtP. Autolysis, as evidenced by reduction in optical density of a stationary-phase culture and concomitant release of intracellular proteins, was greatly reduced when L. lactis MG1363 cells expressed the cell wall-anchored lactococcal proteinase PrtP of the PI-type caseinolytic specificity (PI). On the other hand, lactococcal strains that did not produce the proteinase showed a high level of autolysis, which was also observed when the cells produced the secreted form of PI or a cell wall-anchored proteinase with PIII-type specificity. Autolysis was also increased when MG1363 expressed the cell wall-anchored hybrid PI/PIII-type proteinase PIac. Zymographic analysis of AcmA activity during stationary phase showed that AcmA was quickly degraded by PI and much more slowly by PrtP proteinases with PIII-type and intermediate specificities. Autolysis of L. lactis by AcmA was influenced by the specificity, amount, and location of the lactococcal proteinase. No autolysis was observed when the various proteinases were expressed in an L. lactis acmA deletion mutant, indicating that PrtP itself did not cause lysis of cells. The chain length of a strain was significantly shortened when the strain expressed a cell wall-anchored active proteinase.

Identification of a sodium chloride-regulated promoter in Lactococcus lactis by single-copy chromosomal fusion with a reporter gene

An integration vector, pORI13, was developed to screen in Lactococcus lactis for expression signals induced by changes in the environment and to assay transcriptional activity of genes in single copy. The plasmid carries a promoterless Escherichia coli lacZ gene preceded by a start codon, a lactococcal ribosome binding site, and a multiple cloning site. Chromosomal Sau3AI fragments of L. lactis MG1363 DNA were cloned in pORI13 using a RepA+ E. coli as host. The resulting bank of plasmids was used for Campbell-type integration into the chromosome of L. lactis MG1363. The relatively large size of the chromosomal fragments used increases the chance of retaining complete genes in the targeted region. Screening of integrants in the presence of 0.3 M NaCl resulted in the isolation of a clone (NS3) in which expression of lacZ was dependent on the concentration of chloride ions.

Construction of a food-grade multiple-copy integration system for Lactococcus lactis

A food-grade vector system was developed that allows stable integration of multiple plasmid copies in the chromosome of Lactococcus lactis. The vector consists of the plus origin of replication (Ori+) of the lactococcal plasmid pWV01, the sucrose genes of the lactic acid bacterium Pediococcus pentosaceus PPE1.0 as selectable marker, a multiple-cloning site, and a lactococcal DNA fragment of a well-characterized chromosomal region. The system includes two L. lactis strains, LL108 and LL302, which produce the pWV01 RepA protein essential for replication of the Ori+ vectors. These helper strains allow the construction and isolation of the replicating form of the integration plasmids from a homologous background. Single-crossover integration of the plasmids in L. lactis MG1363 resulted in amplifications to a level of approximately 20 copies/chromosome after selection of the transformants on medium containing sucrose as the only fermentable sugar. The amplifications were stable under selective growth conditions. In glucose-containing medium a limited loss of integrated plasmid copies was detected at a rate of (7.5-15) x 10(-2) copies per generation. One strain, MG124, was isolated that had retained 11 integrated copies after a period of 120 generations of non-selective growth. These results show that the single-cross-over integration system described here represents a simple procedure for the engineering of stable food-grade strains carrying multiple copies of a gene of interest.

A chloride-inducible acid resistance mechanism in Lactococcus lactis and its regulation

Previously, a promoter was identified in Lactococcus lactis that is specifically induced by chloride. Here, we describe the nucleotide sequence and functional analysis of two genes transcribed from this promoter, gadC and gadB. GadC is homologous to putative glutamate-gamma-aminobutyrate antiporters of Escherichia coli and Shigella flexneri and contains 12 putative membrane-spanning domains. GadB shows similarity to glutamate decarboxylases. A L. lactis gadB mutant and a strain that is unable to express both gadB and gadC was more sensitive to low pH than the wild type when NaCl and glutamate were present. Expression of gadCB in L. lactis in the presence of chloride was increased when the culture pH was allowed to decrease to low levels by omitting buffer from the medium, while glutamate also stimulated gadCB expression. Apparently, these genes encode a glutamate-dependent acid resistance mechanism of L. lactis that is optimally active under conditions in which it is needed to maintain viability. Immediately upstream of the chloride-dependent gadCB promoter Pgad, a third gene encodes a protein (GadR) that is homologous to the activator Rgg from Streptococcus gordonii. gadR expression is chloride and glutamate independent. A gadR mutant did not produce the 3kb gadCB mRNA that is found in wild-type cells in the presence of NaCl, indicating that GadR is an activator of the gadCB operon.

A chloride-inducible gene expression cassette and its use in induced lysis of Lactococcus lactis

A chloride-inducible promoter previously isolated from the chromosome of Lactococcus lactis (J. W. Sanders, G. Venema, J. Kok, and K. Leenhouts, Mol. Gen. Genet., in press) was exploited for the inducible expression of homologous and heterologous genes. An expression cassette consisting of the positive-regulator gene gadR, the chloride-inducible promoter Pgad, and the translation initiation signals of gadC was amplified by PCR. The cassette was cloned upstream of Escherichia coli lacZ, the holin-lysin cassette (lytPR) of the lactococcal bacteriophage r1t, and the autolysin gene of L. lactis, acmA. Basal activity of Pgad resulted in a low level of expression of all three proteins. Growth in the presence of 0.5 M NaCl of a strain containing the gadC::lacZ fusion resulted in a 1,500-fold increase of beta-galactosidase activity. The background activity levels of LytPR and AcmA had no deleterious effects on cell growth, but induction of lysin expression by addition of 0.5 M NaCl resulted in inhibition of growth. Lysis was monitored by following the release of the cytoplasmic marker enzyme PepX. Released PepX activity was maximal at 1 day after induction of lytPR expression with 0.1 M NaCl. Induction of acmA expression resulted in slower release of PepX from the cells. The presence of the inducing agent NaCl resulted in the stabilization of osmotically fragile cells.

Design of thermolabile bacteriophage repressor mutants by comparative molecular modeling

Comparative molecular modeling was performed with repressor protein Rro of the temperate Lactococcus lactis bacteriophage r1t using the known 3D-structures of related repressors in order to obtain thermolabile derivatives of Rro. Rro residues presumed to stabilize a nonhomologous but structurally conserved hydrophobic pocket, which was shown to be important for thermostability of the Escherichia coli bacteriophage lambda repressor CI, were randomized. Of the derivatives that exhibited various temperature-sensitive phenotypes, one was shown to hold promise for both fundamental and industrial applications that require the controlled production of (heterologous) proteins in L. lactis.

Autolysis of Lactococcus lactis caused by induced overproduction of its major autolysin, AcmA

The optical density of a culture of lactococcus lactis MG1363 was reduced more than 60% during prolonged stationary phase. Reduction in optical density (autolysis) was almost absent in a culture of an isogenic mutant containing a deletion in the major autolysin gene, acmA. An acmA mutant carrying multiple coples of a plasmid encoding AcmA lysed to a greater extent than the wild-type strain did. Intercellular action of AcmA was shown by mixing end-exponential-phase cultures of an acmA deletion mutant and a tripeptidase (pepT) deletion mutant. PepT, produced by the acmA mutant, was detected in the supernatant of the mixed culture, but no PepT was present in the culture supernatant of the acmA mutant. A plasmid was constructed in which acmA, lacking its own promoter, was placed downstream of the inducible promoter/operator region of the temperate lactococcal bacteriophage r1t. After mitomycin induction of an exponential-phase culture of L. lactis LL302 carrying this plasmid, the cells became subject to autolysis, resulting in the release of intracellular proteins.

Cloning and analysis of the pepV dipeptidase gene of Lactococcus lactis MG1363

The gene pepV, encoding a dipeptidase from Lactococcus lactis subsp. cremoris MG1363, was identified in a genomic library in pUC19 in a peptidase-deficient Escherichia coli strain and subsequently sequenced. PepV of L. lactis is enzymatically active in E. coli and hydrolyzes a broad range of dipeptides but no tri-, tetra-, or larger oligopeptides. Northern (RNA) and primer extension analyses indicate that pepV is a monocistronic transcriptional unit starting 24 bases upstream of the AUG translational start codon. The dipeptidase of L. lactis was shown to be similar to the dipeptidase encoded by pepV of L. delbrueckii subsp. lactis, with 46% identity in the deduced amino acid sequences. A PepV-negative mutant of L. lactis was constructed by single-crossover recombination. Growth of the mutant strain in milk was significantly slower than that of the wild type, but the strains ultimately reached the same final cell densities.

An outbreak of Escherichia coli O157:H7 infections traced to jerky made from deer meat

OBJECTIVE

To investigate a 1995 outbreak of Escherichia coli O157:H7 infections and to assess the safety of meat dehydration methods.

DESIGN

Survey subsequent to routine surveillance report, environmental investigations, and laboratory experimentation.

SETTING

Oregon community.

PARTICIPANTS

Members of an extended household and their social contacts with confirmed or presumptive E coli O157:H7 infections.

RESULTS

A total of 6 confirmed and 5 presumptive cases were identified. Homemade venison jerky was implicated as the source of transmission. E coli O157:H7 with the same distinctive, pulsed-field gel electrophoresis pattern seen in the case isolates was recovered from leftover jerky, uncooked meat from the same deer, a saw used to dismember the carcass, and fragments of the deer hide. In a subsequent survey, E coli O157:H7 was recovered from 3 (9%) of 32 deer fecal pellets collected in nearby forest land. In the laboratory, inoculated venison was dried at several time and temperature combinations, ranging up to 10 hours at 62.8 degrees C. Viable organisms were recovered under all conditions tested.

CONCLUSIONS

Deer can be colonized by E coli O157:H7 and can be a source of human infections. Conditions necessary to ensure the safety of dried meat deserve further review. Game should be handled with the same caution indicated for commercially slaughtered meat.

Rapid and Efficient Purification Method for Small, Hydrophobic, Cationic Bacteriocins: Purification of Lactococcin B and Pediocin PA-1

The bacteriocins lactococcin B and pediocin PA-1 were purified by ethanol precipitation, preparative isoelectric focusing, and ultrafiltration. The procedure reproducibly leads to high final yields in comparison to the generally low yields obtained by column chromatography. Specifically, during isoelectric focusing no loss of activity occurs. The method, in general, should be applicable to small, hydrophobic, cationic bacteriocins.

A general system for generating unlabelled gene replacements in bacterial chromosomes

A general system is described that facilitates gene replacements such that the recombinant strains are not labelled with antibiotic resistance genes. The method is based on the conditional replication of derivatives of the lactococcal plasmid pWV01, which lacks the repA gene encoding the replication initiation protein. Replacement vectors can be constructed in and isolated from gram-positive and gram-negative helper strains that provide RepA in trans. Cointegrate formation of the integration vectors with the chromosome of the target strain is selected by antibiotic resistance. Resolution of the cointegrate structure is identified in the second step of the procedure by the loss of the lacZ reporter gene present in the delivery vector. The second recombination event results either in gene replacement or in restoration of the original copy of the gene. As no antibiotic resistance marker is present in the genome of the mutant the system can be used to introduce multiple mutations in one strain. A feasibility study was performed using Lactococcus lactis and Bacillus subtilis as model organisms. The results indicate that the method should be applicable to any non-essential gene in numerous bacterial species.

Cryptogenic partial epilepsies with anti-GM1 antibodies: a new form of immune-mediated epilepsy?

PURPOSE

We wished to study immune system dysfunction which has been proposed as a potential cause of epilepsy. Epileptogenic action of antibodies directed against GM1 gangliosides was demonstrated in rats, but the potential role of anti-GM1 antibodies in human epilepsy has not yet been studied.

METHODS

We investigated the presence of IgG or IgM anti-GM1 antibodies in the sera of 64 patients with various types of epileptic syndromes: idiopathic generalized epilepsy (IGE) (n = 6), symptomatic or cryptogenic generalized epilepsy (SCGE) (n = 7), symptomatic partial epilepsy (SPE) (n = 26), and cryptogenic partial epilepsy (CPE) (n = 25).

RESULTS

Two patients had elevated titers of IgM anti-GM1 antibodies, one patient had elevated titers of both IgM and IgG anti-GM1 antibodies, and 1 patient had elevated titers of IgG anti-GM1 antibodies. All 4 patients had complex partial seizures (CPS) secondarily generalized, drug resistance, psychiatric disorders, and normal brain imaging. Anti-GM1 antibodies were never associated with IGE, SCGE, or SPE. We compared the reactivity of sera from these patients with the sera from 5 patients with motor neuropathies with conduction block (MNCB) against different gangliosides and concluded that epilepsy sera did not react with the Gal(beta 1-3)GalNAc epitope. Two anti-GM1-positive patients were treated successfully with high-dose intravenous immunoglobulins (IgIV).

CONCLUSIONS

Our findings suggest that detection of anti-GM1 antibodies could allow identification of a subgroup of patients with partial epilepsy associated with an autoimmune response. If anti-GM1 antibodies prove pathogenic, they could be an important prognostic factor for drug resistance and worsening of seizures.

Inducible gene expression and environmentally regulated genes in lactic acid bacteria

Relatively recently, a number of genes and operons have been identified in lactic acid bacteria that are inducible and respond to environmental factors. Some of these genes/operons had been isolated and analysed because of their importance in the fermentation industry and, consequently, their transcription was studied and found to be regulatable. Examples are the lactose operon, the operon for nisin production, and genes in the proteolytic pathway of Lactococcus lactis, as well as xylose metabolism in Lactobacillus pentosus. Some other operons were specifically targetted with the aim to compare their mode of regulation with known regulatory mechanisms in other well-studied bacteria. These studies, dealing with the biosynthesis of histidine, tryptophan, and of the branched chain amino acids in L. lactis, have given new insights in gene regulation and in the occurrence of auxotrophy in these bacteria. Also, nucleotide sequence analyses of a number of lactococcal bacteriophages was recently initiated to, among other things, specifically learn more about regulation of the phage life cycle. Yet another approach in the analysis of regulated genes is the 'random' selection of genetic elements that respond to environmental stimuli and the first of such sequences from lactic acid bacteria have been identified and characterized. The potential of these regulatory elements in fundamental research and practical (industrial) applications will be discussed.

Multidrug resistance mediated by a bacterial homolog of the human multidrug transporter MDR1

Resistance of Lactococcus lactis to cytotoxic compounds shares features with the multidrug resistance phenotype of mammalian tumor cells. Here, we report the gene cloning and functional characterization in Escherichia coli of LmrA, a lactococcal structural and functional homolog of the human multidrug resistance P-glycoprotein MDR1. LmrA is a 590-aa polypeptide that has a putative topology of six alpha-helical transmembrane segments in the N-terminal hydrophobic domain, followed by a hydrophilic domain containing the ATP-binding site. LmrA is similar to each of the two halves of MDR1 and may function as a homodimer. The sequence conservation between LmrA and MDR1 includes particular regions in the transmembrane domains and connecting loops, which, in MDR1 and the MDR1 homologs in other mammalian species, have been implicated as determinants of drug recognition and binding. LmrA and MDR1 extrude a similar spectrum of amphiphilic cationic compounds, and the activity of both systems is reversed by reserpine and verapamil. As LmrA can be functionally expressed in E. coli, it offers a useful prokaryotic model for future studies on the molecular mechanism of MDR1-like multidrug transporters.

Mutational analysis and chemical modification of Cys24 of lactococcin B, a bacteriocin produced by Lactococcus lactis

Using site-directed mutagenesis the single cysteine residue at position 24 of lactococcin B was replaced by all other possible amino acids. Most of these mutant molecules retained bacteriocin activity, with the exception of those in which cysteine was replaced by a positively charged amino acid. This would seem to be in agreement with the authors' earlier observation that treatment of the wild-type molecule with HgCl2 resulted in its inactivation. The factor that causes inactivation of lactococcin B seems to be the introduction of a positive charge at position 24 by HgCl2 rather than oxidation of this residue, as treatment of the bacteriocin with other oxidative chemicals did not interfere with the ability of lactococcin B to dissipate the membrane potential of sensitive cells. Results are also reported which imply that inactive lactococcin B can still bind to its receptor. It can be replaced by an active bacteriocin molecule, resulting in dissipation of the membrane potential.

Fate of peptides in peptidase mutants of Lactococcus lactis

The utilization of exogenous peptides was studied in mutants of Lactococcus lactis in which combinations of the peptidase genes pepN, pepC, pepO, pepX and pepT were deleted. Multiple mutants lacking PepN, PepC, PepT plus PepX could not grow on peptides such as Leu-Gly-Gly, Gly-Phe-Leu, Leu-Gly-Pro, Ala-Pro-Leu and Gly-Leu-Gly-Leu, respectively, indicating that no other peptidases are present to release the essential amino acid Leu. In these mutants, peptides accumulate intracellularly, demonstrating that peptides are translocated as whole entities prior to degradation. The mutant lacking all five peptidases could still grow on Gly-Leu and Tyr-Gly-Gly-Phe-Leu, which confirmed the presence of a dipeptidase and led to the identification of an unknown PepO-like endopeptidase. These studies have also shown that the general aminopeptidases PepN, PepC and PepT have overlapping but not identical specificities and differ in their overall activity towards individual peptides. In contrast, PepX has an unique specificity, because it is the only enzyme which can efficiently degrade Ala-Pro-Leu. The concerted action of peptidases in the breakdown of particular peptides revealed how these substrates are utilized as sources of nitrogen.

The genes for secretion and maturation of lactococcins are located on the chromosome of Lactococcus lactis IL1403

Southern hybridization and PCR analysis were used to show that Lactococcus lactis IL1403, a plasmid-free strain that does not produce bacteriocin, contains genes on its chromosome that are highly homologous to lcnC and lcnD and encode the lactococcin secretion and maturation system. The lcnC and lcnD homologs on the chromosome of IL1403 were interrupted independently by Campbell-type integrations. Both insertion mutants were unable to secrete active lactococcin. Part of the chromosomal lcnC gene was cloned and sequenced. Only a few nucleotide substitutions occurred, compared with the plasmid-encoded lcnC gene, and these did not lead to changes in the deduced amino acid sequence. No genes homologous to those for lactococcin A, B, or M could be detected in IL1403, and the strain does not produce bacteriocin activity.

Multiple-peptidase mutants of Lactococcus lactis are severely impaired in their ability to grow in milk

To examine the contribution of peptidases to the growth of lactococcus lactis in milk, 16 single- and multiple-deletion mutants were constructed. In successive rounds of chromosomal gene replacement mutagenesis, up to all five of the following peptidase genes were inactivated (fivefold mutant): pepX, pepO, pepT, pepC, and pepN. Multiple mutations led to slower growth rates in milk, the general trend being that growth rates decreased when more peptidases were inactivated. The fivefold mutant grew more than 10 times more slowly in milk than the wild-type strain. In one of the fourfold mutants and in the fivefold mutant, the intracellular pools of amino acids were lower than those of the wild type, whereas peptides had accumulated inside the cell. No significant differences in the activities of the cell envelope-associated proteinase and of the oligopeptide transport system were observed. Also, the expression of the peptidases still present in the various mutants was not detectably affected. Thus, the lower growth rates can directly be attributed to the inability of the mutants to degrade casein-derived peptides. These results supply the first direct evidence for the functioning of lactococcal peptidases in the degradation of milk proteins. Furthermore, the study provides critical information about the relative importance of the peptidases for growth in milk, the order of events in the proteolytic pathway, and the regulation of its individual components.

Sequence analysis and molecular characterization of the temperate lactococcal bacteriophage r1t

The temperate lactococcal bacteriophage r1t was isolated from its lysogenic host and its genome was subjected to nucleotide sequence analysis. The linear r1t genome is composed of 33,350 bp and was shown to possess 3' staggered cohesive ends. Fifty open reading frames (ORFs) were identified which are, probably, organized in a life-cycle-specific manner. Nucleotide sequence comparisons, N-terminal amino acid sequencing and functional analyses enabled the assignment of possible functions to a number of DNA sequences and ORFs. In this way, ORFs specifying regulatory proteins, proteins involved in DNA replication, structural proteins, a holin, a lysin, an integrase, and a dUTPase were putatively identified. One ORF seems to be contained within a self-splicing group I intron. In addition, the bacteriophage att site required for site-specific integration into the host chromosome was determined.

Inducible gene expression mediated by a repressor-operator system isolated from Lactococcus lactis bacteriophage r1t

A regulatory region of the temperate Lactococcus lactis bacteriophage r1t chromosome has been cloned and characterized. It encompasses the two divergently oriented genes rro, encoding the phage repressor, and tec. Both genes, of which the transcription start sites have been mapped, are preceded by consensus -35 and -10 promoter sequences. The region contains three 21 bp direct repeats with internal dyad symmetry which probably act as operators. Two of these repeats partially overlap the two promoter sequences. The distant third repeat is located within the tec coding sequence. Gel mobility shift assays demonstrated that Rro specifically binds to this sequence. To study possible transcriptional regulation of the region, a lacZ translational fusion with an open reading frame following tec was constructed. Under conditions that favour the lysogenic life cycle of r1t, beta-galactosidase activity was very low. Expression of the lacZ fusion could be induced 70-fold by the addition of mitomycin C at a concentration which promotes the switch of r1t from the lysogenic to the lytic life cycle. In non-induced cells, promoter activity was repressed by Rro, as a frameshift mutation in rro resulted in constitutive expression of the lacZ gene fusion.

Topology of LcnD, a protein implicated in the transport of bacteriocins from Lactococcus lactis

Four in-frame translational fusions to both the reporter proteins beta-galactosidase and alkaline phosphatase support a topological model of LcnD, a protein implicated in the transport of several bacteriocins from Lactococcus lactis, in which the N-terminal part is located intracellularly and one transmembrane helix spans the cytoplasmic membrane.