Conferring transfer capability by cloning transfer-related genes from the conjugative linear plasmid SAP1

In addition to the usual circular plasmids, linear plasmids also exist in Streptomyces. It is not yet clear what the minimal genetic determinants required for their conjugation. We introduced genes from the conjugative linear plasmid SAP1 into a minimal vector capable of replicating in both circular and linear forms, and monitored plasmid transfer to identify the minimal factors of SAP1 conjugation. Linear vectors were mobilized by adding traA and traB, which have been implicated in the conjugation of SAP1. The addition of ttrA, located near the end of SAP1, greatly increased plasmid transfer efficiency. At least the TtrA protein was essential for the enhancing transfer efficiency. When the vector was maintained in a circular form, only traA and traB were essential for the efficient transfer, which was the case for both RCR and theta replication, without ttrA. These results shed new light on the conjugative transfer system of Streptomyces.