Functional analysis of the plasmid pM4 replicon from Lactobacillus plantarum M4: Determination of the minimal replicon and functionality identification of the putative sso

In order to determine the minimal replicon and the single strand origin (sso) of the plasmid pM4, different fragments of pM4 were amplified by polymerase chain reaction (PCR) and cloned into pBEm, a replication probe vector for Lactobacillus. The deletion analysis results showed that the minimal replicon of pM4 could be determined within a 1280 bp fragment consisting of double strand origin (dso) and rep gene encoding replication protein. Based on plasmid segregation stability assay and its ability to convert single-stranded DNA (ssDNA) to double-stranded DNA (dsDNA) by Southern hybridization, an sso of replication was located at nucleotides −118–92 in the plasmid pM4, about 300 bp upstream of dso. In addition, the host range assay indicated that plasmid pM4 could replicate in L. casei 05–21, L. rhamnosus AS 1.2466T and L. plantarum 05–19 of all the tested Lactobacillus strains. Analysis of the pM4 replicon will allow its use in constructing a food-grade vector for application in food industry.