Doing the conjugative two-step: evidence of recipient autonomy in retrotransfer.

Bidirectional exchange of genetic information, called retrotransfer, during bouts of bacterial conjugation has drawn the interest of those concerned with the risk of releasing genetically engineered microbes, the fluidity of genes among species, and the mechanism of DNA transport between cells. The phenomenon has generated two models in explanation, both of which yield highly testable predictions. The first model, called the one-step, predicts that the flow of genes from recipient bacteria to donor bacteria is mechanistically distinct from, but dependent on, conjugation between donors and recipients. The second model, called the two-step, predicts that the same genetic requirements and mechanistic constraints apply to the process of gene flow from recipients to donors as for gene flow from donors to recipients. The requirement for expression of at least 10 plasmid-encoded genes in recipients, sensitivity of the reverse flow (recipient to donor) to restriction of DNA transferring from the donor, and the requirement of an additional 30-90 min for DNA to flow from recipients back to donors are predictions of the two-step model and directly refute the one-step model. Retrotransfer of genes to donors during conjugation remains genetically and physically indistinguishable from two successive rounds of conjugation between neighbors.