Facilitation of plasmid transfer in Streptococcus pneumoniae by chromosomal homology

The frequency of plasmid establishment in the transformation of Streptococcus pneumoniae by plasmid DNA was increased more than 10-fold when the plasmid carried DNA homologous to the host chromosome. Perfect homology was not necessary for such facilitation; small additions or deletions were tolerated, but extensive deletions in the homologous segment of either plasmid or chromosome reduced or eliminated facilitation. The facilitated plasmid transfer showed a linear dependence on monomeric plasmid concentration rather than the quadratic dependence found in the absence of homology, which indicated that entering plasmid fragments interacted with the chromosome rather than with each other to establish a plasmid replicon. Restriction enzyme cleavage of the plasmid in the nonhomologous segment destroyed its activity, but cleavage in the homologous segment or even enzymatic removal of part of that segment did not prevent plasmid transfer, and plasmids of the original size were established. In facilitated transfer, chromosomal markers (additions and deletions as well as single-site mutations) entered the plasmid with a frequency ranging from 10 to 90% depending on the marker location. Several possible mechanisms for the establishment of plasmids in the presence of chromosomal homology and for the transfer of chromosomal information are considered. They depend on synapsis of the newly entered single-strand plasmid fragment with the host chromosome and subsequent copying of, donation from, or integration into the homologous chromosomal segment. After plasmid establishment, equilibration of donor and chromosomal markers between the chromosome and the plasmid pool, presumably by homologous recombination events, was observed.