Characterization of the cryptic plasmid pTXW from Lactobacillus paracasei TXW

Identification and sequence analysis of two novel cryptic plasmids isolated from the vaginal mucosa of South African women

The vaginal mucosa is dominated by Gram positive, rod shaped lactobacilli which serve as a natural barrier against infection. In both healthy- and bacterial vaginosis (BV)-infected women Lactobacillus crispatus and Lactobacillus jensenii have been found to be the predominant Lactobacillus species. Many studies have been conducted to assess factors influencing lactobacilli dominance in the vaginal microbiome. In the present study two plasmids, pLc4 and pLc17, isolated from vaginal Lactobacillus strains of both healthy and BV-infected women were characterized. The smaller plasmid, pLc4 (4224 bp), was detected in both L. crispatus and L. jensenii strains, while pLc17 was only detected in L. crispatus. Based on its nucleotide sequence pLc4 appears highly novel, with its replication protein having 44% identity to the replication initiation protein of pSMQ173b_03. Phylogenetic analysis with other Rolling Circle Replication plasmids confirmed that pLc4 shows a low degree of similarity to these plasmids. Plasmid pLc17 (16,663 bp) appears to carry both a RCR replicon and a theta replicon, which is rare in naturally occurring plasmids. pLc4 was maintained at a high copy number of 29, while pLc17 appears to be a medium copy number plasmid maintained at 11 copies per chromosome. While sequence analysis is a valuable tool to study cryptic plasmids, further function-based analysis will be required in order to fully elucidate the role of these plasmids within the vaginal milieu.

The qacC Gene Has Recently Spread between Rolling Circle Plasmids of Staphylococcus, Indicative of a Novel Gene Transfer Mechanism

Resistance of Staphylococcus species to quaternary ammonium compounds, frequently used as disinfectants and biocides, can be attributed to qac genes. Most qac gene products belong to the Small Multidrug Resistant (SMR) protein family, and are often encoded by rolling-circle (RC) replicating plasmids. Four classes of SMR-type qac gene families have been described in Staphylococcus species: qacC, qacG, qacJ, and qacH. Within their class, these genes are highly conserved, but qacC genes are extremely conserved, although they are found in variable plasmid backgrounds. The lower degree of sequence identity of these plasmids compared to the strict nucleotide conservation of their qacC means that this gene has recently spread. In the absence of insertion sequences or other genetic elements explaining the mobility, we sought for an explanation of mobilization by sequence comparison. Publically available sequences of qac genes, their flanking genes and the replication gene that is invariably present in RC-plasmids were compared to reconstruct the evolutionary history of these plasmids and to explain the recent spread of qacC. Here we propose a new model that explains how qacC is mobilized and transferred to acceptor RC-plasmids without assistance of other genes, by means of its location in between the Double Strand replication Origin (DSO) and the Single-Strand replication Origin (SSO). The proposed mobilization model of this DSO-qacC-SSO element represents a novel mechanism of gene mobilization in RC-plasmids, which has also been employed by other genes, such as lnuA (conferring lincomycin resistance). The proposed gene mobility has aided to the wide spread of clinically relevant resistance genes in Staphylococcus populations.

Mobilizable Rolling-Circle Replicating Plasmids from Gram-Positive Bacteria: A Low-Cost Conjugative Transfer

Conjugation is a key mechanism for horizontal gene transfer in bacteria. Some plasmids are not self-transmissible but can be mobilized by functions encoded in trans provided by other auxiliary conjugative elements. Although the transfer efficiency of mobilizable plasmids is usually lower than that of conjugative elements, mobilizable plasmidsare more frequently found in nature. In this sense, replication and mobilization can be considered as important mechanisms influencing plasmid promiscuity. Here we review the present available information on two families of small mobilizable plasmids from Gram-positive bacteria that replicate via the rolling-circle mechanism. One of these families, represented by the streptococcal plasmid pMV158, is an interesting model since it contains a specific mobilization module (MOB_V) that is widely distributed among mobilizable plasmids. We discuss a mechanism in which the promiscuity of the pMV158 replicon is based on the presence of two origins of lagging strand synthesis. The current strategies to assess plasmid transfer efficiency as well as to inhibit conjugative plasmid transfer are presented. Some applications of these plasmids as biotechnological tools are also reviewed.

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.