Marker rescue from the Nicotiana tabacum plastid genome using a plastid/Escherichia coli shuttle vector.

We recently reported an 868-bp plastid DNA minicircle, NICE1, that formed during transformation in a transplastomic Nicotiana tabacum line. Shuttle plasmids containing NICE1 sequences were maintained extrachromosomally in plastids and shown to undergo recombination with NICE1 sequences on the plastid genome. To prove the general utility of the shuttle plasmids, we tested whether plastid genes outside the NICE1 region could be rescued in Escherichia coli. The NICE1-based rescue plasmid, pNICER1, carries NICE1 sequences for maintenance in plastids, the ColE1 ori for maintenance in E. coli and a spectinomycin resistance gene (aadA) for selection in both systems. In addition, pNICER1 carries a defective kanamycin resistance gene, kan*, to target the rescue of a functional kanamycin resistance gene, kan, from the recipient plastid genome. pNICER1 was introduced into plastids where recombination could occur between the homologous kan/kan* sequences, and subsequently rescued in E. coli to recover the products of recombination. Based on the expression of kanamycin resistance in E. coli and the analysis of three restriction fragment polymorphisms, recombinant kan genes were recovered at a high frequency. Efficient rescue of kan from the plastid genome in E. coli indicates that NICE1-based plasmids are suitable for rescuing mutations from any part of the plastid genome, expanding the repertoire of genetic tools available for plastid biology.