RESUMO
Commonly, the interbacterial transfer of circular plasmids is initiated by nicking at an internal sequence, oriT, followed by transferring one strand as single-stranded DNA through a type Ⅳ secretion channel on cell membrane. In contrast, Streptomyces conjugative linear plasmids, containing a free 3'-end but a protein-capped 5'-end, can potentially undergo cell-to-cell transfer by transfer of non-nicked DNA. It was reported that circular derivatives of the Streptomyces lividans linear plasmid SLP2, as well as the parental linear plasmid itself can transfer efficiently. And the genetic requirements for such transfer was described. Efficient transfer of plasmid requires six co-transcribed SLP2 genes, encoding a Tra-like DNA translocase, cell wall hydrolase, two cell membrane proteins that interact with an ATP binding protein, and a protein of unknown function. Reduced transfer efficiency of plasmid from SalⅠ R-/M-to Sal Ⅰ R/M hosts argues that transfer of both the circular and linear forms of the plasmid involves double-stranded DNA. These results suggest that conjugal transfer occurs by a similar mechanism for SLP2-derived linear and circular plasmids, and cellular membrane/wall functions in the transfer process.