Homologous recombination between plasmid DNA molecules in maize protoplasts

The requirements for homologous recombination between plasmid DNA molecules have been studied using the PEG (polyethylene glycol)-mediated transformation system of maize (Zea mays L.) protoplasts coupled with the transient expression assay for beta-glucuronidase (GUS). Two plasmids were introduced into maize protoplasts; one plasmid (pB x 26) contained a genomic clone of the Adh1 maize gene; the other plasmid (piGUS) was a promoterless construction containing part of intron A of the Adh1 gene fused to the gusA coding sequence. Thus, the two vectors shared an effective homologous region consisting of a 459 bp (HindIII-PvuII) fragment of the Adh1 intron A sequence. An active gusA fusion gene would result upon homologous recombination between the plasmids within the intron A sequence, and indeed GUS activity was observed in extracts following co-transformation of maize protoplasts with the two plasmids. The presence of recombinant DNA molecules in protoplast DNA isolated 1 day after co-transformation was verified using polymerase chain reactions (PCR) and Southern blots. For efficient homologous recombination, both plasmids had to be linearized. The recombination reaction was induced by restriction of the plasmid molecules either inside the effective homologous region or at the borders of the intron sequence. However, the presence of even small, terminal, nonhomologous sequences at the 3' end of the pB x 26 fragment inhibited the recombination reaction. Also, both ends of the linearized piGUS DNA molecules were involved in the recombination reaction. The results revealed some features of homologous recombination reactions occurring in plant cells which cannot be accommodated by mechanisms postulated for similar reactions in animal system and in lower eukaryotes.