EFFICIENT PLASMID MOBILIZATION BY PIP501 IN LACTOCOCCUS-LACTIS SUBSP LACTIS

pIP501 is a streptococcal conjugative plasmid which can be transmitted among numerous gram-positive strains. To identify a minimal mobilizat ion (mob) locus of pIP501, DNA fragments of pIP501 were cloned into no nconjugative target plasmids and tested for mobilization by pIP501. We show that nonmobilizable plasmids containing a specific fragment of p IP501 are transmitted at high frequencies between Lactococcus lactis s ubsp. lactis strains if transfer (tra) functions are provided in trans by a pIP501 derivative. Independent transfer of the mobilized plasmid was observed in up to 44% of transconjugants. A 2.2-kb segment contai ning mob was sequenced. This DNA segment is characterized by three pal indromes (palI, palII, and palIII) and a 202-amino-acid open reading f rame (ORFX) of unknown function. The smallest DNA fragment conferring high frequency mobilization was localized to a 1.0-kb region (extendin g from pIP501 coordinates 3.60 to 4.60 on the 30.2-kb map) which conta ins pall (DELTAG = -27 kcal/mol [ca. -110,000 J/mol]). A 26-bp sequenc e identical to palI is present on pIP501, upstream of the plasmid copy control region. Further homologies with the palI sequence are also fo und with the related Enterococcus faecalis conjugative plasmid pAMbeta 1. The region containing mob maps outside the previously described seg ment mediating pIP501 conjugation. Our results with recA strains indic ate that the mob site is a hot spot for cointegrate formation.