Sequence analysis of the mobilizable lactococcal plasmid pGdh442 encoding glutamate dehydrogenase activity.

A novel plasmid named pGdh442 had previously been isolated from a plant Lactococcus lactis strain. This plasmid encodes two interesting properties with applications in the dairy industry: a glutamate dehydrogenase activity that stimulates amino acid conversion to aroma compounds, and cadmium/zinc resistance that can be used as a selectable marker. Moreover, this plasmid can be transferred naturally to other strains, but appears to be incompatible with certain other lactococcal plasmids. During this study, the complete sequence of pGdh442 (68 319 bp) was determined and analysed. This plasmid contains 67 ORFs that include 20 IS elements that may have mediated transfer events between L. lactis and other genera living in the same biotope, such as Streptococcus, Pediococcus and Lactobacillus. Even though it is a low-copy-number plasmid, it is relatively stable due to a theta replication mode and the presence of two genes involved in its maintenance system. However, pGdh442 is incompatible with pSK08-derived protease/lactose plasmids because both possess the same replication and partition system. pGdh442 is not self-transmissible, but can be naturally transmitted via mobilization by conjugative elements carried by the chromosome or by other plasmids, such as the 712-type sex factor, which is widely distributed in L. lactis. In addition to several genes already found on other L. lactis plasmids, such as the oligopeptide transport and utilization genes, pGdh442 also carries several genes not yet identified in L. lactis. Finally, it does not carry genes that would trigger concern over its presence in human food.