Plasmid p256 from Lactobacillus plantarum represents a new type of replicon in lactic acid bacteria, and contains a toxin-antitoxin-like plasmid maintenance system.

Lactobacillus plantarum NC7 harbours a single 7.2 kb plasmid called p256. This report describes the complete nucleotide sequence and annotation of p256, as well as the identification of the minimal replicon of the plasmid. Based on sequence features in the unusually small (0.7 kb) minimal replicon, and the absence of a gene for a replication-relevant protein, p256 seems to represent a hitherto unknown type of theta replicon in lactic acid bacteria (LAB), with a relatively low copy-number. In addition, a putative toxin-antitoxin (TA) locus was identified. Experiments with variants of p256 indicated that the TA system was involved in plasmid maintenance. Furthermore, controlled expression of the TA genes stabilized vectors derived from the p256 replicon. To the authors' knowledge, this is the first time a TA locus with a demonstrated plasmid maintenance function has been identified in LAB. Transformation of several LAB with plasmids derived from p256 indicated that it has a narrow host range. Several effective expression vectors based on the p256 replicon have been constructed.