DOING THE CONJUGATIVE 2-STEP - EVIDENCE OF RECIPIENT AUTONOMY IN RETROTRANSFER

Bidirectional exchange of genetic information, called retrotransfer, d uring bouts of bacterial conjugation has drawn the interest of those c oncerned with the risk of releasing genetically engineered microbes, t he 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, ca lled the one-step, predicts that the flow of genes from recipient bact eria to donor bacteria is mechanistically distinct from, but dependent on, conjugation between donors and recipients. The second model, call ed the two-step, predicts that the same genetic requirements and mecha nistic constraints apply to the process of gene flow from recipients t o donors as for gene flow from donors to recipients. The requirement f or expression of at least 10 plasmid-encoded genes in recipients, sens itivity 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. Retrotra nsfer of genes to donors during conjugation remains genetically and ph ysically indistinguishable from two successive rounds of conjugation b etween neighbors.