Molecular Characterization of a Lactococcal Plasmid Reducing the Growth Rate of Host Cells

概要

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Lactococcus lactis subsp. lactis biovar. diacetylactis DRC1 carries more than 6 plasmids, including a 7.4 kb cryptic plasmid, which was designated as pDR1-1. The pDR1-1 plasmid was found to significantly affect the maximum specific growth rate (µmax) of the host cells because of its limiting effect on growth. To investigate the properties of the limiting effect, the entire nucleotide sequence of pDR1-1 was determined. It consisted of 7412 bp, and 6 open reading frames (ORFs) were identified. The first ORF showed a high degree of similarity to a family of replication genes (rep) that are commonly found in lactococcal strains. The rep gene in pDR1-1was followed by a second ORF of unknown function. Directly downstream of the second ORF, a third ORF was found, that showed homology to the S subunit from type I restriction/modification systems. No significant similarity to the contents of the database was found for the other ORFs. PCR analysis was carried out in order to detect pDR1-1 in the other L. lactis strains. The µmax of the pDR1-1-positive strain was the same as that of DRC1. These results suggest that the load of pDR1-1 (or pDR1-1-like plasmid) is a major factor influencing the µmax of DRC1 because of its limiting effect on growth, an effect which is much more pronounced than that produced by the overall load of other coexisting plasmids.
独立行政法人　国際農林水産業研究センターの論文