Molecular and genetic characterization of lactose-metabolic genes of Streptococcus cremoris

Characterization of wild strains ofLactococcus lactissubsp.lactisisolated from Cabrales cheese

TwentyLactococcus lactissubsp.lactisstrains isolated from cheese made from raw milk have been, studied and compared with well known starter strains. Many of them produced acid bacteriocins resembling those of other lactic streptococci and their patterns of antimicrobial agent susceptibility were also similar to previously studied dairy strains. On the basis of their ability to clot milk they could be divided into two groups. Some strains were rapid fermenters, with highβ-phosphogalactosidase and proteolytic activities. The other group had lowβ-phosphogalactosidase activity and coagulated milk slowly, but seemed to have proteolytic activity. All the strains had plasmid DNA in amounts and sizes similar to starter strains. Aβ-phosphogalactosidase probe was constructed with plasmid DNA from aLc. lactissubsp.lactisNCDO 712 derivative. Southern hybridization located homologous sequences in chromosomal and plasmid DNA from the wild isolates that fermented lactose efficiently, but the degree of homology was lower than that observed between starter strains.

Use of a novel mobilizable vector to inactivate the scrA gene of Streptococcus sobrinus by allelic replacement

The virulence factors of the cariogenic bacterium Streptococcus sobrinus have been difficult to assess because of a lack of tools for the genetic manipulation of this organism. The construction of an Escherichia coli-Streptococcus shuttle vector, pDL289, that can be mobilized into S. sobrinus by the conjugative plasmid pAM beta 1 was described in a previous report. The vector contains pVA380-1 for replication and mobilization in streptococci, the pSC101 replicon for maintenance in E. coli, a kanamycin resistance marker that functions in both hosts, and the multiple cloning site and lacZ from pGEM7Zf(-). pDL289 is stable with or without selection in several species of Streptococcus. In this study, a derivative with a deletion in the minus origin of the pVA380-1 component of pDL289 was constructed. This derivative, pDL289 delta 202, was less stable than pDL289 in Streptococcus gordonii Challis, Streptococcus mutans, and S. sobrinus. Both pDL289 and pDL289 delta 202 were mobilizable by pAM beta 1 into S. sobrinus, with frequencies of 3 x 10(-6) and 1 x 10(-7) transconjugants per recipient CFU, respectively. The cloned scrA gene of S. sobrinus 6715-10 coding for the EIISuc of the sucrose-specific phosphoenolpyruvate phosphotransferase system was interrupted by the insertion of a streptococcal spectinomycin resistance gene active in E. coli and streptococci. The interrupted scrA gene was subcloned into both pDL289 and pDL289 delta 202. Each recombinant plasmid was introduced into the DL1 strain of S. gordonii Challis, which was then used as a recipient for the conjugative transfer of pAM beta 1. The latter plasmid was used to mobilize each recombinant plasmid from S. gordonii Challis DL1 to S. sobrinus 6715-10RF. Subsequently, recombinants derived from a double-crossover event were isolated on the basis of resistance to spectinomycin and susceptibility to kanamycin. Recombinational events were confirmed by Southern hybridization, and the inactivation of the EII Suc in double crossovers was confirmed by phosphotransferase system assays. This is the first report of allelic replacement in S. sobrinus.

Genetics of lactose utilization in lactic acid bacteria

Lactose utilization is the primary function of lactic acid bacteria used in industrial dairy fermentations. The mechanism by which lactose is transported determines largely the pathway for the hydrolysis of the internalized disaccharide and the fate of the glucose and galactose moieties. Biochemical and genetic studies have indicated that lactose can be transported via phosphotransferase systems, transport systems dependent on ATP binding cassette proteins, or secondary transport systems including proton symport and lactose-galactose antiport systems. The genetic determinants for the group translocation and secondary transport systems have been identified in lactic acid bacteria and are reviewed here. In many cases the lactose genes are organized into operons or operon-like structures with a modular organization, in which the genes encoding lactose transport are tightly linked to those for lactose hydrolysis. In addition, in some cases the genes involved in the galactose metabolism are linked to or co-transcribed with the lactose genes, suggesting a common evolutionary pathway. The lactose genes show characteristic configurations and very high sequence identity in some phylogenetically distant lactic acid bacteria such as Leuconostoc and Lactobacillus or Lactococcus and Lactobacillus. The significance of these results for the adaptation of lactic acid bacteria to the industrial milk environment in which lactose is the sole energy source is discussed.

Integration and gene replacement in the Lactococcus lactis lac operon: induction of a cryptic phospho-beta-glucosidase in LacG-deficient strains

Insertions, replacement mutations, and deletions were introduced via single or double crossover recombination into the lacE (enzyme IIlac) and lacG (phospho-beta-galactosidase) genes of the Lactococcus lactis chromosomal lacABCDFEGX operon. LacG production was abolished in strains missing the lacG gene or carrying multicopy insertions in the lacE gene that affected expression of the lacG gene. However, these LacG-deficient strains could still ferment lactose slowly and were found to contain an enzymatic activity that hydrolyzed the chromogenic substrate o-nitrophenyl-beta-D-galactopyranoside phosphate. Induction of this phospho-beta-glycohydrolase activity coincided with the appearance of a new 55-kDa protein cross-reacting with anti-LacG antibodies that had a size similar to that of LacG but a higher isoelectric point (pI 5.2) and was not found in wild-type cells during growth on lactose. Since the phospho-beta-glycohydrolase activity and this protein with a pI of 5.2 were highly induced in both mutant and wild-type cells during growth on cellobiose that is likely to be transported via a phosphoenolpyruvate-dependent phosphotransferase system, we propose that this induced activity is a phospho-beta-glucosidase that also hydrolyzes lactose-6-phosphate.

Identification of plasmids in Actinobacillus actinomycetemcomitans and construction of intergeneric shuttle plasmids

A collection of 39 isolates of Actinobacillus actinomycetemcomitans, obtained from laboratories located in 5 different geographical regions of the United States, was examined for the presence of plasmid DNA. Only 2 of the strains examined, designated VT736 and VT745, harbored detectable plasmids. Strain VT736 contained a 1.9 kb plasmid species (pVT736-1) and a larger ( > 30 kb) species (pVT736-2). Both plasmids were detected in the covalently closed circular DNA fraction of dye buoyant density gradients. However, only the smaller plasmid was observed in agarose gels containing plasmid-enriched cell lysates prepared by a rapid screening procedure. Strain VT745 contained a single, 24 kb, plasmid (pVT745) that was observed consistently in plasmid-enriched lysates, as well as in the plasmid band of dye buoyant density gradients. A restriction endonuclease map of pVT736-1 was constructed. The plasmid contained one site each for the enzymes HincII, KpnI and XhoI, located 600 to 700 bp from each other on the pVT736-1 map. HincII-digested pVT736-1 DNA could not be cloned in Escherichia coli. However, intact pVT736-1 digested with KpnI or XhoI could be cloned in E. coli on pUC19 or pGEM7Zf(-), respectively. KpnI-digested pVT736-1 was cloned in both orientations on pUC19, but XhoI-digested pVT736-1 was clonable in only one orientation on pGEM7Zf(-). Each of the 3 types of chimeric plasmid constructs provided a potential A. actinomycetemcomitans/E. coli shuttle plasmid for the development of a genetic transfer system in A. actinomycetemcomitans.

Molecular, genetic, and functional analysis of the basic replicon of pVA380-1, a plasmid of oral streptococcal origin

The 4.2-kb cryptic plasmid pVA380-1 has been used as a vector for the cloning of antibiotic resistance genes directly in streptococci, and in the construction of Escherichia coli/Streptococcus shuttle vectors. The results of subcloning experiments located the basic replicon of pVA380-1 within a 2.5-kb region. The nucleotide base sequence of this region was determined and contained a single complete open reading frame (ORF) encoding a 237-amino-acid peptide with a predicted size of 29 kDa. This peptide and a region of the DNA molecule 5' to the ORF encoding it shared homology with the replication protein and plus origin, respectively, of the Staphylococcus aureus plasmid pUB110. Data from Tn5 mutagenesis and complementation studies indicated that the protein product of the ORF was required for pVA380-1 replication in streptococci. Deletion of a region of the basic replicon distal to the plus origin and ORF produced an unstable derivative, and resulted in the accumulation of single-stranded replicative intermediates, consistent with the loss of a minus origin. All of these results suggest that pVA380-1 replicates by a rolling circle mode, and is most closely related to the pC194 family of single-stranded DNA plasmids.

Cloning and nucleotide base sequence analysis of a spectinomycin adenyltransferase AAD(9) determinant from Enterococcus faecalis

Enterococcus faecalis LDR55, a human clinical isolate, is resistant to tetracycline (Tcr), erythromycin (Emr), and high levels (greater than 2,000 micrograms/ml) of spectinomycin (Spr) but not streptomycin. Filter matings between strain LDR55 and E. faecalis OG1-RF produced transconjugants with the following resistance phenotypes: Tcr Emr Spr, Tcr Emr, Tcr Spr, and Tcr only but never Emr or Spr only. The genetic determinant encoding resistance to spectinomycin was cloned in Streptococcus sanguis Challis from pDL55, a 26-kb plasmid harbored by a Tcr Spr transconjugant. Subcloning experiments yielded a 1.1-kb ClaI-NdeI fragment that encoded very high-level Spr in S. sanguis (10 mg/ml) and Escherichia coli (50 mg/ml). Cell extracts of cultures obtained from Spr strains expressed adenylating activity for spectinomycin but not for streptomycin, indicating that Spr was due to an AAD(9) activity. The nucleotide base sequence of the 1.1-kb ClaI-NdeI fragment contained a single 750-base open reading frame. The protein predicted from the open reading frame consisted of 250 amino acids and had a calculated size of approximately 28,000 daltons, similar to the size estimated from maxicell analysis (29,000 daltons). The deduced amino acid sequence of the streptococcal AAD(9) was compared with that of the AAD(9) encoded by staphylococcal transposon Tn554. The two proteins shared approximately 39% amino acid identity, which was expanded to 53% when conservative amino acid changes were included. When the streptococcal protein was compared with an AAD(3")(9) protein of E. coli, the degrees of identity were 27 and 47%, on the basis of actual amino acids and conservative replacements, respectively. The cloning and nucleotide base sequence analyses of the spectinomycin AAD(9) determinant from E. faecalis that results in high-level Spr when transferred to S. sanguis or E. coli are presented.

In vitro expression of Lac-PTS and tagatose 1,6-bisphosphate aldolase genes from Lactococcus lactis subsp. cremoris plasmid pDI-21

A 4.4-kb EcoR1-EcoR1 DNA fragment from the Lactococcus lactis subsp. cremoris plasmid pDI-21 encoded the tagatose 1,6-bisphosphate (TBP) aldolase gene and the Lac-PTS genes. In vitro transcription-translation using Escherichia coli S30 extract showed the synthesis of 41,000-, 23,000- and 12,000-dalton proteins which correspond to the TBP-aldolase, Lac-PTS enzyme II, and factor III proteins respectively.

Isolation and characterization of Lactococcus lactis subsp. lactis promoters

DNA fragments with promoter activity were isolated from the chromosome of Lactococcus lactis subsp. lactis. For the isolation, a promoter probe vector based on the cat gene was constructed, which allowed direct selection with chloramphenicol in Bacillus subtilis and L. lactis. Four of the putative promoters (P1, P2, P10, and P21) were analyzed further by sequencing, mapping of the 5' end of the mRNA, Northern (RNA blot) hybridization, and chloramphenicol acetyltransferase activity measurements. From these fragments, -10 and -35 regions resembling the consensus Escherichia coli sigma 70 and B. subtilis sigma 43 promoters were identified. Another set of promoters, together with a signal sequence, were also isolated from the same organism. These fragments promoted secretion of TEM beta-lactamase from L. lactis. When the two sets of promoters were compared, it was found that the ones isolated with the cat vector were more efficient (produced more mRNA). By changing the promoter part of the promoter-signal sequence fragment giving the best TEM beta-lactamase secretion into a more efficient one (P2), a 10-fold increase in enzyme production was obtained.

Molecular cloning and expression of a proteinase gene from Lactococcus lactis subsp. cremoris H2 and construction of a new lactococcal vector pFX1

The 6.5 kb HindIII DNA fragment of the Lactococcus lactis subsp. cremoris H2 plasmid pDI21 was cloned into Escherichia coli POP13 with lambda NM1149, and also directly into Lactococcus lactis subsp. lactis 4125 using a newly-constructed broad host-range vector pFX1. Proteinase was expressed in both transformed organisms. The proteinase resembles a PI type since it preferentially degraded beta-casein. The restriction map of the 6.5 kb proteinase gene fragment has minor differences from those of published plasmid proteinase genes. High-efficiency electroporation with pFX1 provides a direct approach for gene cloning in lactococci.

Nucleotide sequence analysis of tetracycline resistance gene tetO from Streptococcus mutans DL5

Streptococcus mutans DL5, isolated from the dental plaque of a pig, was resistant to high levels of streptomycin (Sm, 20 mg/ml), erythromycin (Em, 1 mg/ml), and tetracycline (Tc, greater than 100 micrograms/ml), but contained no detectable plasmid DNA. The Smr and Emr determinants were cloned from cellular DNA on the self-replicating 5-kilobase-pair (kbp) EcoRI fragment of pAM beta 1 and the 4.2-kbp cryptic plasmid pVA380-1, respectively, by transformation of Streptococcus sanguis Challis. Helper plasmid cloning, with a Challis host containing pVA380-1, was required to clone the Tcr determinant of strain DL5 on this vector. A single-colony isolate of the original Tcr clone contained a hybrid plasmid, pDL421, composed of 2.6 kbp of vector DNA and 11.4 kbp of S. mutans DNA. Plasmid pDL421 did not hybridize to plasmids containing the streptococcal Tcr determinants tetL, tetM, and tetN. A shortened derivative of this hybrid plasmid, pDL422, missing a 4.9-kbp HincII fragment from the S. mutans DNA but still encoding Tcr, was obtained by subcloning in S. sanguis Challis. The Tcr gene was located in a 1,917-base-pair open reading frame (ORF) corresponding to a 72-kilodalton protein. The ORF exhibited 99.4% sequence identity with the 1,917-base-pair tetO gene from a strain of Campylobacter coli (W. Sougakoff, B. Papadopoulou, P. Nordmann, and P. Courvalin, FEMS Microbiol. Lett. 44:153-160, 1987). A 1.67-kbp NdeI fragment, internal to the ORF from strain DL5, as well as pDL421 hybridized under stringent conditions to DNA from 10 of 10 Tcr strains of C. coli and Campylobacter jejuni from human and animal sources, but not to DNA from Tcs isolates of these two species.

Molecular cloning and nucleotide sequence of the factor IIIlac gene of Lactobacillus casei

The lactose-specific factor III (FIIIlac of the phosphoenolpyruvate-dependent phosphotransferase system (PTS) was isolated from Lactobacillus casei and purified to homogeneity by conventional protein purification methods. Its apparent native Mr, estimated from steric exclusion chromatography (approx. 39 kDa), and subunit Mr, estimated from sodium dodecyl sulfate-polyacrylamide gels, indicated that it exists as a trimer of identical subunits of 13 kDa. The gene for FIII L. casei lac was cloned into Escherichia coli using the vector pUC18. The coding sequences were contained on an 860-bp BglII-HindIII DNA fragment of the L. casei lactose plasmid, pLZ64. A protein identical in properties to FIII L. casei lac was isolated from clones of E. coli carrying this DNA insert. The nucleotide sequence of the FIII L. casei lac gene was determined by the dideoxy chain-termination technique. The 336-bp open reading frame for FIII L. casei lac was followed by a stem-loop structure, analogous to a Rho-independent terminator. We concluded that the FIII L. casei lac was the terminal gene in what appears to be an operon comprised of the lactose-PTS-P-beta Gal-coding genes. Comparison of the deduced amino acid sequence of FIII L. caseilac with the amino acid sequence of FIII S. aureus lac (derived from peptide sequencing) demonstrated a high degree of homology (49 identical residues and 21 conservative exchanges out of 103 total aa residues). The FIII L. casei lac lacked his82, previously identified as the phosphorylation site of FIII S. aureus. lac His80 was proposed to be the site of histidyl phosphorylation of FIII L. casei lac.

Nucleotide sequence of the beta-D-phosphogalactoside galactohydrolase gene of Lactobacillus casei: comparison to analogous pbg genes of other gram-positive organisms

Lactose metabolism in Lactobacillus casei occurs via phosphoenolpyruvate-dependent phosphotransferase uptake of lactose and subsequent cleavage of lactose-6-phosphate by beta-D-phosphogalactoside galactohydrolase (P-beta Gal). The genes for lactose uptake and P-beta Gal have been shown to be plasmid-associated in L. casei 64H [Chassy et al., Curr. Microbiol. 1 (1978) 141-144]. The cloned P-beta Gal-coding gene (pbg) previously described [Lee et al., J. Bacteriol. 152 (1982) 1138-1146] was subcloned on a 2.9-kb KpnI-Bg/II fragment isolated from pLZ605. Sequence analysis of this fragment revealed an open reading frame of 1422 bp capable of coding for a protein product containing 474 amino acids and having an Mr of 53,989. The L. casei protein showed a high degree of homology to the proteins whose sequence was deduced from the nucleotide sequence of the pbg genes of Staphylococcus aureus and Streptococcus lactis. Because of the significant homologies observed, as reflected in amino acid content as well as predicted structural characteristics of the three proteins, we suggest a common origin for the P-beta Gals of these three organisms.

Isolation and structural analysis of the phospho-beta-galactosidase gene from Streptococcus lactis Z268

A 4.4-kb XhoI fragment of Streptococcus lactis L13 (Z268) lactose plasmid pUCL13, containing the beta-D-phosphogalactoside galactohydrolase (P-beta Gal; EC 3.2.1.85)-coding gene has been cloned in Escherichia coli. Further subcloning and deletion of this fragment allowed localization of the P-beta Gal-coding gene (pbg) on a minimal 1.8-kb segment. Expression of P-beta Gal activity was constitutive and was not regulated by glucose in E. coli. The presence of P-beta Gal activity was correlated with the production of a 56.5-kDa protein in E. coli minicells. The nucleotide sequence of the cloned gene was determined and potential promoter structural elements were identified.

Properties of Lactose Plasmid pLY101 in Lactobacillus casei

A starter strain, Lactobacillus casei C257, was found to carry a lactose plasmid, pLY101. Restriction mapping showed that pLY101 DNA was 68.2 kilobases long. Since a non-lactose-utilizing variant of C257, MSK248, lost phospho-β-galactosidase (P-β-gal) activity and pLY101 DNA had a sequence(s) homologous to the streptococcal fragment including a P-β-gal gene, pLY101 is likely to encode a P-β-gal gene required for lactose metabolism in C257. MSK248 grew in galactose medium at a rate identical to that of C257 and retained phosphoenolpyruvate-dependent phosphotransferase system activity for lactose similar to that of C257. Therefore, the C257 chromosome appears to encode a complete set of genes for the lactose-phosphotransferase system and the predominant galactose metabolic pathway in C257. pLY101 DNA had a sequence homologous to a lactobacillus insertion sequence, ISL 1 , which mapped more than 12 kilobases from the sequence homologous to the streptococcal P-β-gal fragment.

Nucleotide and deduced amino acid sequences of the Staphylococcus aureus phospho-beta-galactosidase gene

We sequenced the Staphylococcus aureus phospho-beta-galactosidase gene. The protein product of this gene consisted of 470 amino acids, giving a molecular weight of 54,557. This gene appears to be transcribed as the terminal sequence on a polycistronic message.