Cloning and sequencing of plasmid pLC494 isolated from human intestinal Lactobacillus casei: construction of an Escherichia coli-Lactobacillus shuttle vector

Characterization of a cryptic plasmid isolated from Lactobacillus casei CP002616 and construction of shuttle vectors based on its replicon

The cryptic plasmid pLC2W was isolated from Lactobacillus casei CP002616. Nucleotide sequence analysis revealed that 4 putative open reading frames (ORF) were responsible for DNA replication. Four Escherichia coli-Lactobacillus shuttle vectors were constructed using different lengths of the pLC2W replicon to identify the shortest functional replicon. The length of the pLC2W replicon did not affect the stability of the plasmids. Green fluorescent protein (GFP) as a reporter was expressed successfully in several lactobacilli using our constructed vectors. The results suggested that the expression vectors pUE-F0GFP and pUE-F1GFP are potential molecular tools for heterologous gene cloning and expression in lactobacilli. Moreover, 2 plasmid-curing methods were used to eliminate pLC2W from L. casei. We detected no difference between L. casei CP002616 and L. casei CP002616 pLC2WΔ-IC (mutant strain cured by plasmid incompatibility method) in production of exopolysaccharide (EPS) or acid. However, EPS and acid production were both reduced in L. casei CP002616 pLC2WΔ-HT (mutant strain cured by high-temperature heat treatment method), demonstrating a difference between these 2 curing methods. Sequence analysis of pLC2W and plasmid curing data suggest that plasmid pLC2W is not involved in EPS synthesis.

Characterization of pMC11, a plasmid with dual origins of replication isolated from Lactobacillus casei MCJ and construction of shuttle vectors with each replicon

Many lactic acid bacteria carry different plasmids, particularly those that replicate via a theta mechanism. Here we describe Lactobacillus casei MCJ(CCTCC AB20130356), a new isolate that contains pMC11, carrying two distinct theta-type replicons. Each replicon contained an iteron in the origin of replication (oriV1 or oriV2) and a gene coding for the replicase (RepA_1 or RepB_1), both of which are essential for plasmid replication. Escherichia coli/Lactobacillus shuttle vectors were constructed with each replicon, yielding pEL5.7 and pEL5.6 that are based on oriV2 and oriV1 replicons, respectively. These plasmids showed distinct properties: pEL5.7 was capable of replicating in L. casei MCJΔ1 and Lactobacillus delbrueckii subsp. lactic LBCH-1 but failed to do so in two other tested lactobacilli strains whereas pEL5.6 replicated in three different strains, including L. casei MCJΔ1, L. casei NJ, Lactobacillus paracasei LPC-37 and L. delbrueckii subsp. lactic LBCH-1. Plasmid stability was studied: pEL5.6 and pEL5.7 were very stably maintained in L. casei, as the loss rate was lower than 1 % per generation. pEL5.7 was also stable in L. delbrueckii subsp. lactic LBCH-1 with the loss rate estimated to be 3 %. These vectors were employed to express a green fluorescent protein (GFP) using the promoter of S-layer protein SlpA from Lactobacillus acidophilus. And a growth-phase regulated expression of GFP was observed in different strains. In conclusion, these shuttle vectors provide efficient genetic tools for DNA cloning and heterologous gene expression in lactobacilli.

Sequencing and analysis of three plasmids from Lactobacillus casei TISTR1341 and development of plasmid-derived Escherichia coli-L. casei shuttle vectors

Pyrosequencing followed by conventional PCR and sequencing was used to determine the complete nucleotide sequence of three plasmids (pRCEID2.9, pRCEID3.2, and pRCEID13.9) from the Lactobacillus casei strain TISTR1341. The plasmid sequences were found to be almost identical, respectively, to those of pLA106, pLA105, and pLA103 from Lactobacillus acidophilus strain TK8912, suggesting that these strains may be related. Sequence analysis and comparison indicated that pRCEID2.9 replicates by a rolling circle (RC) mechanism, while pRCEID3.2 and pRCEID13.9 probably follow a theta-type mode of replication. Replicons of pRCEID2.9 and pRCEID13.9 were used to develop Escherichia coli/L. casei compatible shuttle vectors, which were stably maintained in different genetic backgrounds. Real-time quantitative PCR analysis showed copy numbers of around 4 and 15, respectively, for the pRCEID13.9- and pRCEID2.9-derived shuttle vectors per chromosome equivalent. The functionality of vector pRCEID-LC13.9 was proved by cloning and expressing in L. casei of a green fluorescent protein gene variant from Aequorea victoria under the control of the promoter from a homologous lactate dehydrogenase gene. The new vectors might complement those currently in use for the exploitation of L. casei as a cellular factory and in other biotechnological applications.

Characterization of a small cryptic plasmid from endophytic Pantoea agglomerans and its use in the construction of an expression vector

A circular cryptic plasmid named pPAGA (2,734 bp) was isolated from Pantoea agglomerans strain EGE6 (an endophytic bacterial isolate from eucalyptus). Sequence analysis revealed that the plasmid has a G+C content of 51% and contains four potential ORFs, 238(A), 250(B), 131(C), and 129(D) amino acids in length without homology to known proteins. The shuttle vector pLGM1 was constructed by combining the pPAGA plasmid with pGFPmut3.0 (which harbors a gene encoding green fluorescent protein, GFP), and the resulting construct was used to over-express GFP in E. coli and P. agglomerans cells. GFP production was used to monitor the colonization of strain EGE6gfp in various plant tissues by fluorescence microscopy. Analysis of EGE6gfp colonization showed that 14 days after inoculation, the strain occupied the inner tissue of Eucalyptus grandis roots, preferentially colonizing the xylem vessels of the host plants.

Characterization of a cryptic plasmid pD403 from Lactobacillus plantarum and construction of shuttle vectors based on its replicon

A cryptic plasmid pD403 was isolated from Lactobacillus plantarum D403 derived from fermented dairy products. It was 2,791 bp in size with a G+C content of 37%. Nucleotide sequence analysis revealed two open reading frames, orf1 and orf2. ORF1 (318 amino acids) was identified as a replication protein (RepA). ORF2 (137 amino acids) shared 31% similarity with the transcriptional regulator of Ralstonia pickettii 12D. Functional investigation indicated that ORF2 (Tra) had the ability of improving the transformation efficiency. The origin of replication was predicted, suggesting that pD403 was a rolling-circle-replication (RCR) plasmid. An Escherichia coli/Lactobacillus shuttle vector pCD4032 was constructed based on the pD403 replicon, and proved to be successfully transformed into various lactobacilli including Lactobacillus casei, Lactobacillus plantarum, Lactobacillus fermentum, and Lactobacillus brevis. The transformation efficiencies were ranged from 1.3 x 10(2) to 7 x 10(4) transformants per microgram DNA. Furthermore, an expression vector pCD4033 was developed with the promoter of the lactate dehydrogenase from Lactobacillus delbrueckii 11842. The green fluorescent protein (gfp) as a reporter was expressed successfully in various lactobacilli tested, suggesting that the expression vector pCD4033 had the potential to be used as a molecular tool for heterologous gene cloning and expression in lactobacilli.

Characterization of endogenous plasmids from Lactobacillus salivarius UCC118

The genome of Lactobacillus salivarius UCC118 comprises a 1.83-Mb chromosome, a 242-kb megaplasmid (pMP118), and two smaller plasmids of 20 kb (pSF118-20) and 44 kb (pSF118-44). Annotation and bioinformatic analyses suggest that both of the smaller plasmids replicate by a theta replication mechanism. Furthermore, it appears that they are transmissible, although neither possesses a complete set of conjugation genes. Plasmid pSF118-20 encodes a toxin-antitoxin system composed of pemI and pemK homologs, and this plasmid could be cured when PemI was produced in trans. The minimal replicon of pSF118-20 was determined by deletion analysis. Shuttle vector derivatives of pSF118-20 were generated that included the replication region (pLS203) and the replication region plus mobilization genes (pLS208). The plasmid pLS203 was stably maintained without selection in Lactobacillus plantarum, Lactobacillus fermentum, and the pSF118-20-cured derivative strain of L. salivarius UCC118 (strain LS201). Cloning in pLS203 of genes encoding luciferase and green fluorescent protein, and expression from a constitutive L. salivarius promoter, demonstrated the utility of this vector for the expression of heterologous genes in Lactobacillus. This study thus expands the knowledge base and vector repertoire of probiotic lactobacilli.

Characterisation of pSMA23, a 3.5 kbp plasmid of Lactobacillus casei, and application for heterologous expression in Lactobacillus

The complete nucleotide sequence of plasmid pSMA23 isolated from Lactobacillus casei A23 was determined. Plasmid pSMA23 is a 3497bp circular molecule with a G+C content of 38.18%. Four putative open reading frames were identified. Based on homology, two orfs were identified as genes encoding replication initiation (Rep) and mobilisation (Mob) protein, respectively. Various regulatory regions like promoters, ribosome binding site (RBS), transcriptional terminators were deduced from the sequences of rep and mob. The origin of replication (dso) was predicted. Shuttle vectors pL142 and pL157 were constructed for Escherichia coli and Lactobacillus using rep gene and ori of pSMA23 for replication in Lactobacillus, the ori of the commercial vector pBluescript SkII+ for replication in E. coli, and the erythromycin and chloramphenicol resistance genes of pE194 and pC194, respectively, as selection markers. Transformants of E. coli and Lactobacillus were obtained on media containing erythromycin and chloramphenicol, respectively, suggesting expression of the ermC and cat194 genes in both organisms. The shsp gene of plasmid pSt04 of Streptococcus thermophilus encoding a small heat shock protein and the Lactobacillus plantarum cbh gene encoding conjugated bile salts hydrolase were cloned and successfully expressed in the heterologous host Lb. casei LK1 with the aid of pSMA23-derived vectors.

Expression of citrate permease gene of plasmid pCM1 isolated from Lactococcus lactis subsp. lactis biovar diacetylactis NIAI N-7 in Lactobacillus casei L-49-4

Recombinant vector pJLECit (8,232 bp) was constructed using citrate permease gene contained in the 3,919-bp fragment of plasmid pCM1 (8,280 bp) isolated from Lactococcus lactis subsp. lactis biovar diacetylactis NIAI N-7, repA and ori from pLU1, and pMB1 ori and the erythromycin resistance gene from pJIR418. Lactobacillus casei L-49-4 (plasmid-free mutant of strain L-49) harboring the constructed pJLECit converted citrate into diacetyl/acetoin. Citrate uptake rate of resting cells was the highest at pH 5.5 and 10 mM citrate concentration. Diacetyl formation activity by the cell-free extracts of Lb. casei L-49-4 (pJLECit) grown in de Man-Rogosa-Sharpe (MRS) broth was higher than that of cells grown in MRS broth without citrate. On the other hand, diacetyl reductase activity of cells grown in MRS broth was lower than that of cells grown in MRS broth without citrate.