Effect of genomic location on horizontal transfer of a recombinant gene cassette between Pseudomonas strains in the rhizosphere and spermosphere of barley seedlings

The use of genetically engineered bacteria in natural environments constitu tes a risk of transfer of recombinant DNA to the indigenous bacteria. Howev er, chromosomal genes are believed to be less likely to transfer than genes on mobilizable and conjugative plasmids. To study this assumption, horizon tal transfer of a recombinant gene cassette inserted into the chromosome of a Pseudomonas strutzeri strain, into a mobilizable plasmid (pAGM42), and i nto a conjugative plasmid (pKJK5) isolated from barley rhizosphere was inve stigated. Horizontal transfer efficiencies of the gene cassette inserted in to a conjugative plasmid was 8.20 x 10(-3) transconjugants/(donors x recipi ents)(1/2) in the rhizosphere and 4.57 x 10(-2) transconjugants/(donors x r ecipients)(1/2) in the spermosphere. Mobilization of the plasmid pAGM42 by the plasmids RP4 and pKJK5 was also detected at high levels in the microcos ms, transfer efficiencies were up to 4.36 x 10(-3) transconjugants/(donors x recipients)(1/2). Transfer of chromosomal encoded genes could not be dete cted in the microcosms by conjugation or transformation. However, transform ation did occur by using the same bacterial strains under laboratory condit ions. The rhizosphere and especially the spermosphere thus proved to be hot spot environments providing favorable conditions for gene transfer by mobi lization and conjugation, but these environments did not support transforma tion at a detectable level.