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Cloning and partial sequencing of the proteinase gene complex from Lactococcus lactis subsp. lactis UC317.

The proteinase genes from Lactococcus lactis subsp. lactis UC317 were identified on a plasmid, pCI310, which is a deletion derivative of a cointegrate between pCI301, the 75 kb Lac Prt plasmid from UC317 and the 38.5 kb cryptic plasmid from that strain. The prt genes were cloned using a replacement cloning strategy whereby fragments from pCI310 were exchanged with the equivalent fragments in pNZ521, which contains the cloned proteinase genes from L. lactis subsp. lactis SK112. This generated two plasmids which encoded a cell-envelope-associated and a secreted proteinase, respectively. Specific regions of the UC317 structural prtP gene known to encode seven of the amino acids essential for substrate cleavage specificity were sequenced and compared with the known sequences of prt genes from L. lactis strains SK112, Wg2 and NCDO763. In spite of various differences that were detected in the nucleotide sequence of this region, it appears that these seven amino acids in strains UC317 and NCDO763 are identical, and represent a combination of three of the amino acids from SK112 and four from Wg2. These results indicate that the UC317 proteinase is a natural hybrid of the SK112 and Wg2 proteinases.

Amino Acid Sequence↗

Phage DNA synthesis and host DNA degradation in the life cycle of Lactococcus lactis bacteriophage c6A.

Bacteriophage c6A is a lytic phage that infects strains of Lactococcus lactis. Infection of L. lactis strain C6 resulted in inhibition of culture growth within 10 min, mature intracellular phage particles appeared after 17.5 min, and cell lysis occurred after 25 min. A culture of strain C6 carrying 3H-labelled DNA was infected with c6A, and the fate of the radiolabel was monitored. The results showed that degradation of host cell DNA began within 6 min of infection and that the breakdown products were incorporated into progeny c6A DNA. Quantitative DNA hybridizations indicated that synthesis of phage DNA began within 6 min of infection and continued at an approximately constant rate throughout the latent period.

Bacteriophages↗

Cloning and sequence analysis of the dnaK gene region of Lactococcus lactis subsp. lactis.

A 5.4 kb HindIII fragment of Lactococcus lactis subsp. lactis was identified using a homologous dnaK probe generated by PCR and cloned in Escherichia coli. Upstream sequences were generated by inverse PCR. The two cloned fragments partially overlapped, and sequencing of 5915 bp revealed the presence of four open reading frames in the order orf1-grpE-dnaK-orf4. orf1 encodes a 39 kDa protein of unknown function which shows considerable sequence homology with the Orf39 and Orfa proteins of Bacillus subtilis and Clostridium acetobutylicum, respectively. The downstream ORFs showed high homology to the grpE and dnaK genes of other prokaryotes. The DnaK protein has a characteristic 24-amino-acid deletion exhibited by all the known DnaK proteins of Gram-positive species. In many bacteria the dnaK and dnaJ genes are found as part of the same operon. The L. lactis dnaK operon is unusual in that the dnaK gene is followed by a putative transcription terminator and a fourth large ORF which shares no homology with the dnaJ genes of other bacteria but has a small degree of homology with various membrane proteins. Vegetative promoter sequences are found upstream of both orf1 and orf4. A 12 bp inverted repeat is found upstream of the putative promoter of orf1 and an 8 bp inverted repeat is found between this promoter and the orf1 initiation codon. These repeats are thought to be involved in regulation of the heat-shock genes. The DnaK homologue is induced approximately 3-fold on heat shock at 42 degrees C.

Amino Acid Sequence↗

Cloning, nucleotide sequence and expression in Streptomyces lividans and Escherichia coli of pabB from Lactococcus lactis subsp. lactis NCDO 496.

A gene (pabB) encoding the aminase activity of p-aminobenzoate (PABA) synthase in Lactococcus lactis subsp. lactis was cloned in pIJ41 and expressed in Streptomyces lividans strains defective in PABA biosynthesis. Expression of the gene was associated with a 1.2 kb deletion between the aph promoter and the cloning site in pIJ41. Subcloning in pBR322 and expression in Escherichia coli AB3295 of the cloned L. lactis DNA fragment localized the pabB-complementing gene in a 1.9 kb segment. The nucleotide sequence of this segment contained a 1410 bp open reading frame encoding a 470-amino-acid polypeptide of 50937 Da. The deduced amino acid sequence showed substantial similarity to those reported for PabB and TrpE from several organisms. Synonymous codon usage reflected the low G + C content in the genomic DNA of L. lactis subsp. lactis, and therefore differed markedly from the preferred usage in the S. lividans host. The cloned heterologous pabB DNA was expressed in amounts that allowed accumulation of excreted PABA in cultures of S. lividans transformants.

Amino Acid Sequence↗

Physiological and genetic regulation of rRNA synthesis in Lactococcus.

The macromolecular composition of Lactococcus was regulated by growth rate in the same general way as that of less fastidious bacteria such as Escherichia coli and Salmonella typhimurium. The ratios of RNA:DNA and RNA:protein increased approximately threefold over a 13.5-fold increase in growth rate, whereas the ratio of DNA:protein remained approximately constant. Using reporter genes fused to a DNA fragment of a cloned lactococcal rRNA operon, promoter activity was located upstream of the 16S rRNA structural gene. This DNA fragment had some characteristics typical of a rrn promoter in E. coli. Two consensus promoter sequences P1 and P2 were located 296 and 157 bp, respectively, upstream of the start of the 16S rRNA gene. Between P2 and the start of the 16S rRNA gene, sequences were identified with typical anti-termination motifs characteristic of E. coli rrn promoter regions. A putative transcription terminator sequence was identified downstream of the 5S rRNA gene and putative primary RNA transcript processing sites at both ends of the lactococcal rRNA operon were also noted.

Amino Acid Sequence↗

A Lactococcus lactis gene encodes a membrane protein with putative ATPase activity that is homologous to the essential Escherichia coli ftsH gene product.

A gene, encoding a protein homologous to an essential Escherichia coli protein, FtsH, was identified adjacent to the hpt gene and the trnA operon in the Gram-positive bacterium Lactococcus lactis. The deduced amino acid sequence of the gene product showed full-length similarity to FtsH of E. coli, Yme1p of Saccharomyces cerevisiae and a conserved region found in a new family of putative ATPases. In-frame fusions of L. lactis ftsH and phoA1 in E. coli, and immunodetection of the L. lactis FtsH protein in cell fractions using anti-E. coli FtsH serum showed that L. lactis ftsH was expressed and encodes a membrane protein. When contained on a high copy number plasmid, the L. lactis ftsH gene complemented the lethality of a delta ftsH3::kan mutation in E. coli at 37 degrees C and below, indicating that the L. lactis ftsH gene can functionally replace the E. coli ftsH gene to some extent. The resulting E. coli strain showed temperature sensitivity and salt sensitivity. A L. lactis mutant with an insertion into ftsH was salt-, heat- and cold-sensitive. These results suggest that FtsH is somehow involved in stress responses. Southern hybridization analysis indicated that genes homologous to ftsH of L. lactis were also present in Bacillus subtilis, and several Lactobacillus and Leuconostoc species, suggesting high conservation of ftsH in bacterial species.

ATP-Dependent Proteases↗

Purification and characterization of an endopeptidase from Lactococcus lactis subsp. cremoris SK11.

An endopeptidase has been purified from Lactococcus lactis subsp. cremoris SK11. The enzyme is a 70 kDa monomer, strongly inhibited by the metalloproteinase inhibitors 1,10-phenanthroline and phosphoramidon but relatively insensitive to EDTA. It is not significantly inhibited by the thiol enzyme inhibitor p-chloromercuribenzoate nor by the serine protease inhibitor phenylmethylsulphonyl fluoride. The action of the endopeptidase in catalysing the hydrolysis of several peptide hormones has been studied and the hydrolysis products identified by sequence analysis. The enzyme catalyses hydrolysis of peptide bonds in which a hydrophobic amino acid (most commonly a Phe or Leu) residue occupies the position immediately C-terminal to the hydrolysed bond. It thus has a specificity very similar to that of thermolysin. Two of the oligopeptides produced during the early stages of beta-casein digestion by the lactococcal cell-wall proteinases were hydrolysed by the endopeptidase, the others were resistant to hydrolysis. Cell fractionation studies have shown that the distribution of endopeptidase activity between the different cell fractions is the same as that of the intracellular marker enzyme fructose bisphosphate aldolase, and thus indicate a cytoplasmic location for the enzyme. These observations argue against a role for this enzyme in the early stages of casein breakdown by the lactococcal proteolytic system.

Amino Acid Sequence↗

The Lactococcus lactis triosephosphate isomerase gene, tpi, is monocistronic.

Triosephosphate isomerase (EC 5.3.1.1) from Lactococcus lactis was purified to electrophoretic homogeneity. Approximately 3 mg purified enzyme (specific activity 3300 U mg-1) was obtained from 70 g (wet wt) cells. In solution, triosephosphate isomerase (pI 4.0-4.4) was observed to exist as a homodimer (M(r) 57,000) of noncovalently linked subunits. The sequence of the first 37 amino acid residues from the NH2-terminus were determined by step-wise Edman degradation. This sequence, and that of a region conserved in all known bacterial triosephosphate isomerases, was used to design oligonucleotide primers for the synthesis of a lactococcal tpi probe by PCR. The probe was used to isolate a molecular clone of tpi from a lambda GEM11 library of L. lactis LM0230 DNA. The nucleotide sequence of tpi predicted a protein of 252 amino acids with the same NH2-terminal sequence as that determined for the purified enzyme and a subunit M(r) of 26,802 after removal of the NH2-terminal methionine. Escherichia coli cells harbouring a plasmid containing tpi had 15-fold higher triosephosphate isomerase activity than isogenic plasmid-free cells, confirming the identity of the cloned gene. Northern analysis of L. lactis LM0230 RNA showed that a 900 base transcript hybridized with tpi. The 5' end of the transcript was determined by primer extension analysis to be a G located 65 bp upstream from the tpi start codon. These transcript analyses indicated that in L. lactis, tpi is expressed on a monocistronic transcript. Nucleotide sequencing indicated that the DNA adjacent to tpi did not encode another Embden-Meyerhoff-Parnas pathway enzyme. The location of tpi on the L. lactis DL11 chromosome map was determined to be between map coordinates 1.818 and 1.978.

Amino Acid Sequence↗

Product formation and phosphoglucomutase activities in Lactococcus lactis: cloning and characterization of a novel phosphoglucomutase gene.

Maltose metabolism in Lactococcus lactis involves the conversion of beta-glucose 1-phosphate to glucose 6-phosphate, a reaction which is reversibly catalysed by a maltose-inducible and glucose-repressible beta-phosphoglucomutase (beta-PGM). The gene encoding beta-PGM (pgmB) was cloned from a genomic library of L. lactis using antibodies. The nucleotide sequence of a 5695 bp fragment was determined and six ORFs, including the pgmB gene, were found. The gene expressed a polypeptide with a calculated molecular mass of 24210 Da, which is in agreement with the molecular mass of the purified beta-PGM (25 kDa). A short sequence at the N-terminus was found to be similar to known metal-binding domains. The expression of beta-PGM in L lactis was found to be induced also by trehalose and sucrose, and repressed by lactose in the growth medium. This indicates that beta-PGM does not serve solely to degrade maltose, but that it is also involved in the metabolism of other carbohydrates. The specific activity of beta-PGM during fermentation was dependent on the maltose concentration in the medium. The maximum specific activity of beta-PGM increased by a factor of 4.6, and the specific growth rate by a factor of 7, when the maltose concentration was raised from 0.8 to 11.0 g l-1. Furthermore, a higher amount of lactate produced relative to formate, acetate and ethanol was observed when the initial maltose concentration in the medium was increased. The specific activity of alpha-PGM responded similarly to beta-PGM, but the magnitude of the response was lower. Preferential sugar utilization and alpha- and beta-PGM suppression was observed when L. lactis was grown on the substrate combinations glucose and maltose, or lactose and maltose; maltose was the least-preferred sugar. In contrast, galactose and maltose were utilized concurrently and both PGM activities were high throughout the fermentation.

Amino Acid Sequence↗

Clustered organization and transcriptional analysis of a family of five csp genes of Lactococcus lactis MG1363.

A family of genes encoding cold-shock proteins, named cspA, cspB, cspC, cspD and cspE, was cloned and sequenced from Lactococcus lactis MG1363. The genes cspA and cspB and the genes cspC and cspD are located in tandem repeats, an organization of csp genes that has never been encountered before. The five genes encode small (7.1-7.6 kDa) proteins with high mutual sequence identities (up to 85%) and high identities (about 45-65%) with the major cold-shock proteins from Escherichia coli (CspA) and Bacillus subtilis (CspB). Northern-blot analysis revealed single transcripts of about 300 nucleotides for each csp gene and showed that cspA, cspB, cspC and cspD mRNA levels were strongly increased upon cold shock to 10 degrees C (about 10-, 40-, 10- and 30-fold compared to 30 degrees C, respectively), whereas the cspE mRNA level was not increased. The expression of the cold-induced csp genes was highest in the 6-8 h lag phase after cold shock. A differential expression in time, in which cspA and cspC were maximally expressed at 2 h and cspB and cspD at 4 h after cold shock, was observed. The -35 and -10 regions of the five promoters were identified and transcriptional start sites were mapped in each case by primer extension at different temperatures which confirmed that regulation takes place at the transcriptional level. Significant differences were observed between the 5'-untranslated leader regions of the four cold-induced csp genes and the corresponding region of the non-cold-induced cspE gene.

5' Untranslated Regions↗

Transcription of the trp operon in Lactococcus lactis is controlled by antitermination in the leader region.

The regulatory functions of the leader region preceding the Lactococcus lactis trp operon have been studied by mutagenesis analysis. This leader presents striking similarity to 'T-box' leaders found upstream of many Gram-positive aminoacyl-tRNA synthetase genes and some amino acid biosynthesis operons, which are controlled by antitermination through interaction of the leader transcript with cognate uncharged tRNA. A region of the L. lactis leader transcript also contains a series of (G/U) AG repeats which, in Bacillus, are involved in the binding of the trp RNA-binding protein (TRAP) which controls trp transcription. A screen was developed for the isolation of regulatory mutants affected in the leader region. All spontaneous mutants contained deletions; point mutations were only obtained after UV-induced mutagenesis. All mutations affected the putative transcription terminator upstream of the trp operon, demonstrating that trp is indeed controlled by transcription antitermination.

Base Sequence↗

Analysis of the role of 7 kDa cold-shock proteins of Lactococcus lactis MG1363 in cryoprotection.

Low-temperature adaptation and cryoprotection were studied in the lactic acid bacterium Lactococcus lactis MG1363. An approximately 100-fold increased survival after freezing was observed when cells were shocked to 10 degrees C for 4 h compared to mid-exponential-phase cells grown at 30 degrees C, indicating an active protection against freezing. Using two-dimensional gel electrophoresis a group of 7 kDa cold-induced proteins (CSPs) was identified that corresponds to a previously described family of csp genes of L. lactis MG1363 (Wouters et al., 1998, Microbiology 144, 2885-2893). The 7 kDa CSPs appeared to be the most strongly induced proteins upon cold shock to 10 degrees C. Northern blotting and two-dimensional gel electrophoresis showed that the csp genes were maximally expressed at 10 degrees C, while induction was lower at 20 and 4 degrees C. However, pre-incubation at 20 and 4 degrees C, as well as stationary-phase conditions, also induced cryoprotection (approx. 30-, 130- and 20-fold, respectively, compared to 30 degrees C mid-exponential phase). For all treatments leading to an increased freeze survival (exposure to 4, 10 and 20 degrees C and stationary-phase conditions), increased levels of three proteins (26, 43 and 45 kDa) were observed for which a role in cryoprotection might be suggested. Increased freeze survival coincides with increased CSP expression, except for stationary-phase conditions. However, the level of observed freeze protection does not directly correlate with the csp gene expression levels. In addition, for the first time specific overproduction of a CSP in relation to freeze survival was studied. This revealed that L. lactis cells overproducing CspD at 30 degrees C show a 2-10-fold increased survival after freezing compared to control cells. This indicates that the 7 kDa cold-shock protein CspD may enhance the survival capacity after freezing but that other factors supply additional cryoprotection.

Adaptation, Physiological↗

Six putative two-component regulatory systems isolated from Lactococcus lactis subsp. cremoris MG1363.

The genetic elements specifying six putative two-component regulatory systems (2CSs) were identified on the chromosome of Lactococcus lactis MG1363. These 2CSs appear to represent distinct loci, each containing a histidine kinase and response-regulator-encoding gene pair. Transcriptional analysis of the six 2CSs was performed either by generating transcriptional fusions to a reporter gene or by primer extension. Two of the systems appeared to be expressed constitutively at a high level, whilst the remaining four exhibited growth-phase-dependent expression. Insertional mutagenesis established that the two constitutively expressed 2CSs are necessary for normal cell growth and/or survival. Mutational analysis of the remaining four systems revealed that they are implicated in susceptibility to extreme pH, osmotic or oxidative conditions, or the regulation of phosphatase activity in L. lactis.

Amino Acid Sequence↗

ctsR of Lactococcus lactis encodes a negative regulator of clp gene expression.

Bacteria undergo a complex programme of differential gene expression in response to stress. In Bacillus subtilis, it was recently shown that CtsR, a negative transcriptional regulator, mediates stress-induced expression of components of the Clp protease complex. In this study, a gene was identified in the Gram-positive bacterium Lactococcus lactis that encodes a 17 kDa product with 38% identity to the CtsR protein of B. subtilis. By Northern analyses it was found that in a L. lactis strain carrying a large internal deletion of ctsR, including the region encoding a putative helix-turn-helix motif, the amounts of clpC, clpP, clpB and clpE mRNAs were increased 3-8-fold compared to those present in wild-type L. lactis MG1363. In another ctsR mutant strain in which only one-third of CtsR was deleted, leaving the putative DNA-binding domain and the C-terminal 29 amino acids intact, only minor derepression of clp gene expression was observed and, furthermore, all the clp genes were still induced by heat. These results indicate that the amino acids of CtsR involved in temperature sensing are located either close to the DNA-binding domain or in the C-terminal part of the protein. Thus, in L. lactis in addition to B. subtilis, CtsR is a key regulator of heat-shock-induced gene expression, suggesting that the presence of CtsR-homologous DNA-binding sites observed in many Gram-positive bacteria reflects functional heat-shock regulatory systems.

Adenosine Triphosphatases↗

Lactococcus lactis LM0230 contains a single aminotransferase involved in aspartate biosynthesis, which is essential for growth in milk.

Amino acid aminotransferases (ATases), which catalyse the last biosynthetic step of many amino acids, may have important physiological functions in Lactococcus lactis during growth in milk. In this study, the aspartate ATase gene (aspC) from L. lactis LM0230 was cloned by complementation into Escherichia coli DL39. One chromosomal fragment putatively encoding aspC was partially sequenced. A 1179 bp ORF was identified which could encode for a 393 aa, 43.2 kDa protein. The deduced amino acid sequence had high identity to other AspC sequences in GenBank and is a member of the Igamma family of ATases. Substrate-specificity studies suggested that the lactococcal AspC has ATase activity only with aspartic acid (Asp). An internal deletion was introduced into the L. lactis chromosomal copy of aspC by homologous recombination. The wild-type and mutant strain grew similarly in defined media containing all 20 amino acids and did not grow in minimal media unless supplemented with asparagine (Asn). The mutant strain was also unable to grow in or significantly acidify milk unless supplemented with Asp or Asn. These results suggest that only one lactococcal ATase is involved in the conversion of oxaloacetate to Asp, and Asp biosynthesis is required for the growth of L. lactis LM0230 in milk.

Animals↗

Genetic localization and regulation of the maltose phosphorylase gene, malP, in Lactococcus lactis.

Maltose phosphorylase (MP) from Lactococcus lactis was purified and the corresponding gene was cloned and expressed in Escherichia coli. The isoelectric point of the pure enzyme was determined to be 7.0. According to zymogram analysis and SDS-PAGE, the native MP was shown to be a monomeric enzyme with a molecular mass of 75 kDa. A polyclonal antiserum was produced to assess the regulation of the gene encoding MP in LC: lactis. According to immunoblot analysis, synthesis of the enzyme was markedly repressed by both glucose and lactose in the growth medium. When the lactococci were cultivated in the presence of other sugars, including maltose, trehalose or galactose, there was a pronounced expression of the MP gene. In addition, when the cells were grown in media without any added sugar, there was also pronounced expression of the enzyme, according to immunoblot analysis and specific activity data. These results indicated that no particular sugar specifically induces the gene encoding MP. However, an effect of glucose on MP expression was demonstrated by performing fermentations in the presence of both maltose and glucose. When glucose was added to maltose-grown lactococci in the mid-exponential growth phase, both the specific activity and amount of MP per millilitre of cell extract decreased rapidly. The genetic locus for the MP gene was found to be in the vicinity of the region encoding a possible regulator belonging to the LacI-GalR family of transcriptional regulators. Furthermore, this genetic location was separated from the previously characterized maltose-inducible and glucose-repressible beta-phosphoglucomutase (beta-PGM) gene. The different genetic loci for the genes encoding MP and beta-PGM explains the different gene regulation behaviour.

Amino Acid Sequence↗

Catabolism of mannitol in Lactococcus lactis MG1363 and a mutant defective in lactate dehydrogenase.

Mannitol metabolism in Lactococcus lactis MG1363 and in a derivative strain deficient in lactate dehydrogenase (LDH(d)) was characterized. Both strains had the ability to grow on mannitol as an energy source, although this polyol was a poorer substrate for growth than glucose. When compared to glucose, the metabolism of mannitol caused an NADH burden due to formation of an additional NADH molecule at the reaction catalysed by mannitol-1-phosphate dehydrogenase (Mtl1PDH). This resulted in a prominent accumulation of mannitol 1-phosphate (Mtl1P) both in growing and resting cells, suggesting the existence of a severe bottleneck at Mtl1PDH. Growth on mannitol induced the activity of Mtl1PDH in both the LDH(d) and MG1363 strains. The lower accumulation of Mtl1P in mannitol-grown cells when compared to glucose-grown LDH(d) cells, as monitored by in vivo (13)C-NMR, reflects this induction. A clear shift towards the production of ethanol was observed on mannitol, indicating pressure to regenerate NAD(+) when this substrate was used. A strategy to obtain a mannitol-overproducing strain is proposed.

Biomass↗

Regulation of 2-deoxyglucose phosphate accumulation in Lactococcus lactis vesicles by metabolite-activated, ATP-dependent phosphorylation of serine-46 in HPr of the phosphotransferase system.

Lactococcus lactis takes up glucose and the nonmetabolizable glucose analogue 2-deoxyglucose (2DG) via the phosphotransferase system and extrudes the accumulated sugar phosphates in a process apparently dependent on a cytoplasmic sugar-phosphate phosphatase. Uptake of 2DG into L. lactis vesicles was shown to be dependent on an energy source, effectively provided by intravesicular phosphoenolpyruvate (PEP). 2DG phosphate (2DG-P) accumulation in these vesicles was not inhibited, and preaccumulated 2DG-P was not released from them, upon electroporation of fructose 1,6-diphosphate (FDP), gluconate 6-phosphate or 2-phosphoglycerate into the vesicles. Intravesicular but not extravesicular wild-type HPr of Bacillus subtilis alone stimulated uptake, but in the presence of any one of these metabolites, it prevented accumulation of 2DG-P. Intravesicular H15A mutant HPr inhibited uptake and allowed further inhibition of 2DG-P accumulation in the presence of the intravesicular metabolites. Intravesicular S46A mutant HPr stimulated uptake but could not promote inhibition in the presence of the phosphorylated metabolites. The S46D mutant HPr protein promoted regulation, even in the absence of a metabolite. The Vmax but not the Km value for 2DG uptake was affected. Accumulation of the natural, metabolizable substrates of the lactose, glucose, mannose and ribose permeases was inhibited by wild-type HPr in the presence of FDP or by S46D mutant HPr. The results establish that HPr serine phosphorylation by the ATP-dependent, metabolite-activated HPr kinase selectively determines the levels of sugar accumulation via the glucose and lactose permeases in L. lactis.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphate↗