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PubMed1997

A highly conserved DNA replication module from Streptococcus thermophilus phages is similar in sequence and topology to a module from Lactococcus lactis phages.

A highly conserved DNA region extending over 5 kb was observed in Streptococcus thermophilus bacteriophages. Comparative sequencing of one temperate and 26 virulent phages demonstrated in the most extreme case an 18% aa difference for a predicted protein, while the majority of the phages showed fewer, if any aa changes. The relative degree of aa conservation was not homogeneous over the DNA segment investigated. Sequence analysis of the conserved segment revealed genes possibly involved in DNA transactions. Three predicted proteins (orf 233, 443, and 382 gene product (gp)) showed nucleoside triphosphate binding motifs. Orf 443 gp showed in addition a DEAH box motif, characteristically found in a subgroup of helicases, and a variant zinc finger motif known from a phage T7 helicase/primase. Tree analysis classified orf 443 gp as a distant member of the helicase superfamily. Orf 382 gp showed similarity to putative plasmid DNA primases. Downstream of orf 382 a noncoding repeat region was identified that showed similarity to a putative minus origin from a cryptic S. thermophilus plasmid. Four predicted proteins showed not only high degrees of aa identity (34 to 63%) with proteins from Lactococcus lactis phages, but their genes showed a similar topological organization. We interpret this as evidence for a horizontal gene transfer event between phages of the two bacterial genera in the distant past.

Amino Acid Sequence↗

PubMed2001

Comparative genomics of lactococcal phages: insight from the complete genome sequence of Lactococcus lactis phage BK5-T.

Lactococcus lactis phage BK5-T and Streptococcus thermophilus phage Sfi21, two cos-site temperate Siphoviridae with 40-kb genomes, share an identical genome organization, sequence similarity at the amino acid level over about half of their genomes, and nucleotide sequence identity of 60% over the DNA packaging and head morphogenesis modules. Siphoviridae with similarly organized genomes and substantial protein sequence similarity were identified in several genera of low-GC-content Gram-positive bacteria. These phages demonstrated a gradient of relatedness ranging from nucleotide sequence similarity to protein sequence similarity to gene map similarity over the DNA packaging and head morphogenesis modules. Interestingly, the degree of relatedness was correlated with the evolutionary distance separating their bacterial hosts. These observations suggest elements of vertical evolution in phages. The structural genes from BK5-T shared no sequence relationships with corresponding genes/proteins from lactococcal phages belonging to distinct lactococcal phage species, including phage sk1 (phage species 936) that showed a closely related gene map. Despite a clearly distinct genome organization, lactococcal phages sk1 and c2 showed nine sequence-related proteins. Over the early gene cluster phage BK5-T shared nine regions of high nucleotide sequence similarity, covering at most two adjacent genes, with lactococcal phage r1t (phage species P335). Over the structural genes, the closest relatives of phage r1t were not lactococcal phages belonging to other phage species, but Siphoviridae from Mycobacteria (high-GC-content Gram-positive bacteria). Evidence for recent horizontal gene transfer between distinct phage species was obtained for dairy phages, but these transfers were limited to phages infecting the same bacterial host species.

Computational Biology↗

PubMed1995

A study of the substrate specificity of aminopeptidase N from Lactococcus lactis subsp. cremoris Wg2.

A systematic study was made of the ability of aminopeptidase N from Lactococcus lactis subsp. cremoris Wg2 to hydrolyse different peptide substrates. The enzyme showed a marked preference for substrates containing arginine as the N-terminal residue but, to a lesser extent, was also capable of cleaving other residues such as lysine and leucine. There was a tendency for the activity to increase with the hydrophobicity index of the C-terminal residue of dipeptide substrates. It was also observed that the enzyme tended to have higher affinities but lower Vmax values for tripeptides with hydrophobic C-terminal residues. The values determined for Km and Vmax increased with chain length for oligopeptides of the general formula Lys-Phe-(Gly)n, the optimum, as determined from Vmax/Km, being when n = 4. Typical Km values for the most effective substrates were in the range 0.2-0.6 mM.

Amino Acid Sequence↗

PubMed1994

Action of a cell-envelope proteinase (CEPIII-type) from Lactococcus lactis subsp. cremoris AM1 on bovine kappa-casein.

The specificity of the cell-envelope proteinase (CEPIII-type) from Lactococcus lactis subsp. cremoris AM1 in its action on bovine kappa-casein was studied. A 4-h digest (pH 6.2, 15 degrees C) of kappa-casein was made with the purified proteinase. The pH-4.6 soluble fraction, representing more than 95% of the whole hydrolysate, was ultrafiltered to obtain a high-molecular-mass (HMM) and a low-molecular-mass (LMM) fraction, which were separately further purified by electrophoretic and chromatographic techniques. Isolated HMM and LMM products were identified by amino acid analysis, end-group determination and mass spectrometry. On-line HPLC/mass spectrometry was also used for the separation of an LMM peptide mixture and the identification of its components. The HMM products formed were the fragments 1-160, 1-151, 1-95 and 1-79 of kappa-casein, whereas the main LMM products found were the 161-169 and 152-160 fragments. The enzyme specificity was concluded to be primarily directed towards the C-terminal region of the substrate molecule by cleavage of the 160-161 and 151-152 peptide bonds. Two minor LMM products were identified as the fragments 96-104 and 103-106, indicating additional cleavage at positions 102-103, 104-105 and 106-107 of the sequence. Also several peptide bonds within the 161-169 sequence were found to be subject to secondary cleavage by the proteinase. From electrophoretic and identification data it is concluded that the lactococcal CEPI, CEPIII and several mixed-type proteinases all act on the peptide bonds at positions 79-80 and 95-96.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗

PubMed1991

Specificity of a cell-envelope-located proteinase (PIII-type) from Lactococcus lactis subsp. cremoris AM1 in its action on bovine beta-casein.

The action of the cell-envelope proteinase (PIII-type) from Lactococcus lactis ssp. cremoris AM1 on bovine beta-casein was studied. The results were compared with those obtained earlier with (PI-type) proteinases from the cell envelope of other L. lactis strains. From a 4-h digest (pH 6.2; 15 degrees C) of beta-casein made with the PIII-type proteinase, 24 peptides were isolated and purified by selective precipitation followed by semi-preparative reversed-phase HPLC. Altogether, these peptides accounted for the preferential splitting of 16 peptide bonds in beta-casein by the PIII-type proteinase. In nine cases the primary cleavage site (P1-P'1) was a Glx-X or X-Glx peptide bond. In ten cases at least one large hydrophobic residue (Met, Leu, Tyr, Phe) formed part of the cleavable bond. The P2-P3 and/or P'2-P'3 regions of the substrate consisted of hydrophobic and/or negatively charged side chains or of side chains potentially involved in hydrogen bonds. Nine of the peptide bonds split were reported previously to be also susceptible to cleavage by PI-type proteinases, although the kinetics may be different. The PIII-type proteinase shows a broader specificity in its initial cleavage of beta-casein than does the PI-type.

Amino Acid Sequence↗

PubMed1995

The occurrence of two intracellular oligoendopeptidases in Lactococcus lactis and their significance for peptide conversion in cheese.

Two intracellular oligopeptide-preferring endopeptidases have been detected in Lactococcus lactis. A neutral thermolysin-like oligoendopeptidase (NOP) has been purified to homogeneity and an alkaline oligoendopeptidase has been partially purified. The specificity of the oligoendopeptidases towards important intermediary cheese peptides, produced by chymosin action on the caseins, clearly differs from that of the cell-envelope proteinase (CEP). NOP is active under conditions prevailing in cheese and contributes to initial proteolysis in a young cheese. It probably plays a crucial role in the degradation of an important bitter peptide in cheese, the beta-casein 193-209 fragment. The relatively low activity of the alkaline endopeptidase is further suppressed in cheese by the highly competitive actions of NOP and CEP.

Amino Acid Sequence↗

PubMed1995

Lactococcin A overexpression in a Lactococcus lactis subsp. lactis transformant containing a Tn5 insertion in the lcnD gene.

Lactococcin A production in lactococci has recently been linked to a signal-sequence-independent secretory system consisting of a four-gene cluster. Lactococcus lactis subsp. lactis LLM23L-A1 has been obtained after Tn5 mutagenesis of pLLM23, a plasmid containing the gene cluster responsible for lactococcin A production. In contrast to other Tn5-generated mutants, strain LLM23L-Al exhibited a 12-fold increase in lactococcin A production. Overproduction of lactococcin A was not linked to an increased pLLM23 copy number. Restriction-enzyme analysis indicated the site of Tn5 insertion to be at the 3' end of lcnD, and upstream of the lcnA structural gene. From DNA sequencing, the Tn5 insertion was located -79 bp upstream of the transcription start site of the lcnA and lciA genes, eliminating eight amino acids from the C-terminal end of lactococcin D. Northern blots revealed overproduction of a 500-base transcript in strain LLM23L-A1, which corresponded to that predicted from the positions of the lactococcin A operon transcriptional start site and the termination structures. This result suggests that the overproduction of lactococcin A in strain LLM23L-A1 is at the transcriptional level and provides further impetus for elucidating the complete regulatory mechanism for lactococcin A expression.

Bacterial Proteins↗

PubMed1992

Antisense RNA directed against the major capsid protein of Lactococcus lactis subsp. cremoris bacteriophage 4-1 confers partial resistance to the host.

Antisense RNA targeted against the major capsid protein (MCP) of Lactococcus lactis subsp. cremoris bacteriophage F4-1 reduced bacteriophage replication by up to 50%. The region containing the mcp gene was oriented to transcribe the antisense strand using a L. lactis subsp. cremoris Wg2 promoter. The size of the mcp insert transcribed affected the level of bacteriophage inhibition and the greatest level of inhibition was achieved using a 301-bp fragment from the 5' end of the mcp. Antisense mcp RNA constructs were stable and did not alter the endogenous plasmid profile in the host, L. lactis subsp. cremoris F4-1. There were, however, some adverse effects on the host during the stationary phase as exhibited by a decline in cell density.

Bacteriophages↗

PubMed1992

Lysozyme expression in Lactococcus lactis.

Three lysozyme-encoding genes, one of eukaryotic and two of prokaryotic origin, were expressed in Lactococcus lactis subsp. lactis. Hen egg white lysozyme (HEL) could be detected in L. lactis lysates by Western blotting. No lysozyme activity was observed, however, presumably because of the absence of correctly formed disulphide bonds in the L. lactis product. The functionally related lysozymes of the E. coli bacteriophages T4 and lambda were produced as biologically active proteins in L. lactis. In both cases, the highest expression levels were obtained using configurations in which the bacteriophage lysozyme genes had been translationally coupled to a short open reading frame of lactococcal origin. Both enzymes, like HEL, may prevent the growth of food-spoilage bacteria.

Bacteriophage lambda↗

PubMed1991

Action of a cell wall proteinase from Lactococcus lactis subsp. cremoris SK11 on bovine alpha s1-casein.

The cell wall-associated proteinase from Lactococcus lactis subsp. cremoris SK11 was partially purified and incubated with alpha s1-casein for various times up to 48 h. Sixteen trifluoroacetic acid-soluble oligopeptide hydrolysis products were identified by determination of the amino acid sequence. Eleven of these oligopeptides originated from the 78-residue sequence comprising the C-terminal region of alpha s1-casein and were present among the products after the first 60 min of digestion. Three oligopeptides from the N-terminal region and two others from the central region of the alpha s1-casein sequence were also present among the early digestion products although in smaller amounts than most of the oligopeptides from the C-terminal region. No clear consensus sequence of amino acid residues surrounding the cleavage sites could be identified.

Amino Acid Sequence↗

PubMed1992

Batch cultures of recombinant Lactococcus lactis subsp. lactis in a stirred fermentor. I. Effect of plasmid content on bacterial growth and on genetic stability in pure cultures.

The effect of plasmid introduction into Lactococcus lactis subsp. lactis IL2661 on the growth of this strain and on plasmid stability was studied in pure batch cultures. The plasmids used (coding for erythromycin or chloramphenicol resistance) were the following: pIL205 (42 kb), pIL252 (4.6 kb, 6-9 copies), pIL253 (4.8 kb, 45-85 copies) and pE194 (inserted in the chromosome). Growth and acidification of L. lactis subsp. lactis IL2661 were similar to those of the derived recombinant lactococci. The maximal population at the end of the fermentation (9 h) was about 1.1 +/- 0.3 x 10(10) cfu/ml, and maximal growth rate 0.92 +/- 0.07 h-1. Growth yield and lactic acid concentrations were 3.9 +/- 0.8 x 10(11) cfu/g lactose consumed and 25.6 +/- 2.3 g/l, respectively. Different levels of plasmid stability were detected. Plasmid pE194, and plasmids pIL252 and pIL253 in the absence of pIL205, were stable after 10 h of culture. A slight loss (1-2%) of pIL205 was observed in all strains. In the presence of pIL205, plasmids pIL252 and pIL253 were maintained in only 56-95% of the cells. This result suggested an incompatibility between pIL205 and pIL252 or pIL253.

Biotechnology↗

PubMed1992

Batch cultures of recombinant Lactococcus lactis subsp. lactis in a stirred fermentor. II. Plasmid transfer in mixed cultures.

The transfer of plasmids was studied in a stirred fermentor in the course of mixed batch cultures combining recombinant strains of Lactococcus lactis subsp. lactis (donor strains) with L. lactis subsp. lactis CNRZ 268M3 (recipient strain). Donor strains contained one or two of the following plasmids (coding for erythromycin or chloramphenicol resistance): pIL205 (self-transmissible), pIL252, pIL253 (non-transmissible but mobilizable by pIL205, respectively small and large copy number) and pE194 (inserted in the chromosome). Only self-transmissible plasmid pIL205 was transferred, with frequencies ranging from 10(-7) to 10(-8) after 12 h of fermentation. These frequencies were 60-400 times lower than in unstirred M17 broth and 100,000 times lower than on agar medium. In the latter case, non-transmissible plasmids pIL252 and pIL253 were mobilized by pIL205 with a frequency of about 10(-5) - 10(-6).

Biotechnology↗

PubMed1990

Characterization of a plasmid involved with cointegrate formation and lactose metabolism in Lactococcus lactis subsp. lactis OZS1.

A 55 kilobase (kb) plasmid (pOZS550) in the non-clumping Lactococcus lactis subsp. lactis strain OZS1 carrying genes for lactose metabolism was characterised. A mobilizable cointegrate plasmid which is formed between pOZS550 and pOZS448 carries the necessary information for conjugation and transfer. Cointegrate formation was found to involve an insertional element located on pOZS550. The insertion sequence was found to be identical to ISS1 located on pSK08 in the clumping L. lactis subsp. lactis strain ML3. Restriction maps of pOZS550 and pSK08 were similar suggesting a close ancestral relationship, although pSK08, in addition to the lactose metabolism genes, expressed genes for proteinase activity and cell clumping, which were not expressed by pOZS550, and carried two copies of ISS1 compared to one on pOZS550. Furthermore, hybridization of the 18 base pair inverted repeat, of the insertion sequence, with various L. lactis subsp. lactis strains and two L. lactis subsp. cremoris strains showed moderate to strong hybridization to one plasmid in each organism.

Base Sequence↗

PubMed1991

Distance-dependent translational coupling and interference in Lactococcus lactis.

The possibility of raising the expression level of a heterologous gene in Lactococcus lactis by exploiting the principle of translational coupling was investigated. For this purpose, the Escherichia coli lacZ gene was transcriptionally fused to a short open reading frame (ORF) of lactococcal origin. A Shine-Dalgarno (SD) sequence was introduced at the boundary of the two ORFs. In a series of otherwise identical plasmids, the relative positions of the translational stop codon of the upstream ORF and the translational start codon of the downstream ORF (lacZ) were varied. The expression of lacZ gradually increased as the stop and start codons were placed in closer proximity. A concomitant switch from translational interference to translational coupling was observed. Best results were obtained with partially overlapping stop and start codons. It is concluded that the principle of translational coupling offers good possibilities to increase the level of heterologous gene expression in L. lactis.

Amino Acid Sequence↗

PubMed1992

Isolation of purine auxotrophic mutants of Lactococcus lactis and characterization of the gene hpt encoding hypoxanthine guanine phosphoribosyltransferase.

Five purine auxotrophic mutants of Lactococcus lactis were isolated. L. lactis was capable of converting adenine, guanine and hypoxanthine to AMP, GMP and IMP, respectively, indicating the existence of adenine phosphoribosyltransferase (APRT) and hypoxanthine guanine phosphoribosyltransferase (HGPRT) activities. A 1.3 kb DNA fragment from L. lactis was cloned by complementation of the hpt mutation in Escherichia coli. Introduction of this fragment into L. lactis resulted in an increase in HGPRT activity. In vitro transcription and translation analysis showed that the fragment coded for a polypeptide with M(r) of 22,000. The nucleotide sequence of this hpt gene was determined.

Adenine Phosphoribosyltransferase↗

PubMed1993

Functional analysis of the Lactococcus lactis usp45 secretion signal in the secretion of a homologous proteinase and a heterologous alpha-amylase.

The ups45 gene encodes the major extracellular protein from Lactococcus lactis. The deduced sequence of the 27 residue leader peptide revealed the tripartite characteristics of a signal peptide. This leader peptide directed the efficient secretion of the homologous proteinase (PrtP) in L. lactis, indicating that the putative signal peptide of PrtP can be replaced by the 27 residue Usp45 leader peptide. In addition, the 27 residue leader peptide could be used to secrete the Bacillus stearothermophilus alpha-amylase, encoded by the amyS gene. Fusion of the usp45 promoter region and various parts of the leader sequence to an amyS gene devoid of its signal sequence, showed that in Escherichia coli the first 19, 20, and 27 residues of the Usp45 leader are able to direct alpha-amylase secretion. In L. lactis the shorter signal peptides did not result in secretion of alpha-amylase, providing experimental evidence for the hypothesis that gram-positive bacteria require a longer signal peptide for secretion than gram-negative organisms.

Amino Acid Sequence↗

PubMed1995

Genetic aspects of aromatic amino acid biosynthesis in Lactococcus lactis.

Polymerase chain reaction (PCR) primers designed from a multiple alignment of predicted amino acid sequences from bacterial aroA genes were used to amplify a fragment of Lactococcus lactis DNA. An 8 kb fragment was then cloned from a lambda library and the DNA sequence of a 4.4 kb region determined. This region was found to contain the genes tyrA, aroA, aroK, and pheA, which are involved in aromatic amino acid biosynthesis and folate metabolism. TyrA has been shown to be secreted and AroK also has a signal sequence, suggesting that these proteins have a secondary function, possibly in the transport of amino acids. The aroA gene from L. lactis has been shown to complement an E. coli mutant strain deficient in this gene. The arrangement of genes involved in aromatic amino acid biosynthesis in L. lactis appears to differ from that in other organisms.

3-Phosphoshikimate 1-Carboxyvinyltransferase↗

PubMed1995

Citrate utilization gene cluster of the Lactococcus lactis biovar diacetylactis: organization and regulation of expression.

The transport of citrate in Lactococcus lactis biovar diacetylactis is mediated by the citrate permease P. This polypeptide is encoded by the citP gene carried by plasmid pCIT264. In this report, we characterize the citP transcript, identify a cluster of two genes cotranscribed with citP and describe their post-transcriptional regulation. The transcriptional promoter is located 1500 nucleotides upstream of the citP gene and the transcriptional terminator is positioned next to the 3'-end of this gene. The DNA sequence was determined of the region upstream of the citP gene, including the promoter. Two partially overlapping open reading frames, citQ and citR were identified, which could encode polypeptides of 3.9 and 13 kDa respectively. These two genes, together with citP, constitute the cit cluster. Moreover, an IS-like element located between the cit promoter and the citQ open reading frame was identified. This element includes an open reading frame ORF1, which could encode a 33 kDa polypeptide. A translational fusion between the citP and a cat reporter gene showed that translation of citR and citP is coupled, and regulated by CitR. The cit mRNA was subjected to specific cleavage after addition of rifampicin to the bacterial cultures. We propose that expression of the cit cluster is controlled at the post-transcriptional level by mRNA processing at a putative complex secondary structure and by translational repression mediated by CitR.

Amino Acid Sequence↗