Plasmids are autonomous episomally replicating genetic elements, which carry backbone genes important for the replication and maintenance within their host, and accessory genes that might confer an advantage to their host under specific selective pressure in its ecological niche. The genome of dairy isolate L. lactis subsp. lactis bv. diacetylactis S50 was sequenced using the PacBio SMRT Cell Seq-RSII platform and revealed to possess one of the largest plasmidomes among L. lactis strains studied so far, harboring six plasmids: pS6 (5553 bp), pS7a (7308 bp), pS7b (7266 bp), pS19 (19,027 bp), pS74 (74,256 bp) and pS127 (127,002 bp) in total representing 8.9% of genome size (240,412 bp). Based on predicted plasmid replication proteins and origins it appears that all six plasmids replicate via the theta-type mechanism. The two the largest plasmids (pS74 and pS127), carry a number of genes known to be important for growth and survival in the dairy environment. These genes encode technological functions such as bacteriocin production, protein degradation, magnesium and cobalt/nickel transporters, selenium binding, exopolysaccharides (EPS) production, bacteriophage and stress resistance. Beside genes for replication, the small plasmids (pS6, pS7a, pS7a, and pS19) also carry genes important for mobilization and host survival such as type I restriction-modification (R-M) system, metal transporters, enzymes and transcriptional regulators. All plasmids in S50 strain are mobilizable, containing an oriT sequences, while pS127 is self-conjugative and allows for mobilization of the other plasmids. Small plasmids are prone to structural and segregational instability, while pS127 appeared to be segregationally stable thanks to the possession of two partition systems. The main characteristic of plasmid pS74 is EPS production, while plasmid pS127 is characterized by proteinase and multiple bacteriocins, tra locus, phage abortive systems and metal transporters. In addition to LcnA and LcnB, plasmid pS127 encodes several bacteriocin-pheromone molecules and a new bacteriocin named LcnS50, with narrow spectrum of action limited to lactococci, that has been successfully cloned and heterologously expressed.

质粒是自主的游离复制基因元件，其携带对于其宿主内复制和维持重要的骨架基因，以及在其生态位中的特定选择压力下可能赋予宿主优势的辅助基因。乳品分离乳杆菌亚种的基因组。球菌BV。使用PacBio SMRT Cell Seq-RSII平台对二乙酰基actis S50进行了测序，结果显示它具有乳酸乳球菌中最大的质粒组之一迄今为止研究的菌株，共有六个质粒：pS6（5553 bp），pS7a（7308 bp），pS7b（7266 bp），pS19（19,027 bp），pS74（74,256 bp）和pS127（127,002 bp），共占8.9％基因组大小（240,412 bp）。根据预测的质粒复制蛋白和来源，似乎所有六个质粒均通过theta型机制。两种最大的质粒（pS74和pS127）带有许多已知对奶牛环境中的生长和存活至关重要的基因。这些基因编码技术功能，例如细菌素产生，蛋白质降解，镁和钴/镍转运蛋白，硒结合，胞外多糖（EPS）产生，噬菌体和抗逆性。除了用于复制的基因外，小的质粒（pS6，pS7a，pS7a和pS19）还携带了对动员和宿主存活至关重要的基因，例如I型限制性修饰（RM）系统，金属转运蛋白，酶和转录调节因子。S50菌株中的所有质粒都是可动员的，包含一个oriTpS127是自缀合的，并允许其他质粒的动员。小质粒容易出现结构和分离不稳定性，而pS127由于具有两个分配系统而显得分离稳定。质粒pS74的主要特征是EPS生产，而质粒pS127的特征是蛋白酶和细菌素多，TRA轨迹，噬菌体流产系统和金属转运蛋白。除LcnA和LcnB外，质粒pS127还编码了数个细菌素-信息素分子和一个名为LcnS50的新细菌素，其作用谱仅限于乳球菌，已成功克隆并异源表达。