Theta replication of the lactococcal plasmid pWVO2.

pWVO2 is a 3.8 kb narrow-host-range plasmid from Lactococcus lactis ssp. cremoris Wg2, which does not replicate in Bacillus subtilis or Escherichia coli. Single-stranded pWVO2 DNA was not observed in lactococcal cells, indicating that this plasmid does not replicate via a rolling-circle mechanism. The sequence of pWVO2 neither showed the structural organization typical for rolling-circle plasmids, nor were sequence similarities with known rolling-circle plasmids present. By 2-D agarose gel electrophoresis of replication intermediates, it was shown that pWVO2 replicates via a theta mechanism. This is the first proof for the existence of theta-replicating plasmids in lactococci. The pWVO2 minimal replicon is strongly related to that of several other lactococcal plasmid replicons. It contains one open reading frame encoding the replication protein, which is preceded by a 22 bp sequence tandemly repeated three and a half times. Further upstream is another 10 bp direct repeat present in an A/T-rich sequence. This structural organization resembles that of several iteron-containing theta-type plasmids from E. coli. Derivatives of pWVO2 were stably maintained in L. lactis and are good candidates for the development of stable food-grade cloning vectors for this organism.