On the question of the integration of exogenous bacterial DNA into plant DNA.

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Abstract
Extensive studies with pea, tomato and barley failed to confirm the evidence presented by previous investigators for integration or replication of exogenously applied bacterial DNA [Pseudomonas aeruginosa] in these plants. Labeled DNA of buoyant density in CsCl intermediate between that of high density donor bacterial DNA and of plant DNA was never observed with axenic plants. Intermediate peaks, similar to those used as evidence for recombination by earlier investigators, were observed only when the plants were contaminated with bacteria. Plant DNA prepared by a published procedure was contamined with unidentified impurities. Such DNA was partially protected from the action of DNase and produced aberrant banding patterns in CsCl after shearing. Much of the published evidence for integration of foreign DNA in plants is based upon experiments with plant DNA prepared by this procedure. Contamination is the likely explanation for what was interpreted as evidence for integration.