Homeologous plastid DNA transformation in tobacco is mediated by multiple recombination events

Efficient plastid transformation has been achieved in Nicotiana tabacum usi ng cloned plastid DNA of Solanum nigrum carrying mutations conferring spect inomycin and streptomycin resistance. The use of the incompletely homologou s (homeologous) Solanum plastid DNA as donor resulted in a Nicotiana plasti d transformation frequency comparable with that of other experiments where completely homologous plastid DNA was introduced. Physical mapping and nucl eotide sequence analysis of the targeted plastid DNA region in the transfor mants demonstrated efficient site-specific integration of the 7.8-kb Solanu m plastid DNA and the exclusion of the vector DNA. The integration of the c loned Solanum plastid DNA into the Nicotiana plastid genome involved multip le recombination events as revealed by the presence of discontinuous tr-act s of Solanum-specific sequences that were interspersed between Nicotiana-sp ecific markers. Marked position effects resulted in very frequent cointegra tion of the nonselected peripheral donor markers located adjacent to the ve ctor DNA. Data presented here on the efficiency and features of homeologous plastid DNA recombination are consistent with the existence of an active R ccA-mediated, but a diminished mismatch, recombination/repair system in hig her-plant plastids.