SUMMARY: We have screened strains of for spontaneous mutants showing constitutive transfer of the nopaline Ti plasmid pTiC58 during conjugation. The Ti plasmid derivatives obtained could be transferred not only to but also to cells. The Ti plasmid cannot survive as a freely replicating plasmid in , but it can occasionally integrate into the chromosome. However, insertion in tandem of plasmids carrying fd replication origins (pfd plasmids) into the T-DNA provides an indicator for all transfer events into cells, providing fd gene 2 protein is present in these cells. This viral protein causes the excision of one copy of the pfd plasmid and allows its propagation in the host cell. By using this specially designed Ti plasmid, which was also made constitutive in transfer functions, we found plasmid exchange among strains and between and cells to be equally efficient. A Ti plasmid with repressed transfer functions was transferred to with a rate similar to the low frequency at which it was transferred to The expression of transfer functions of plasmid RP4 either in or in did not increase the transfer of the Ti plasmid into cells, nor did the addition of acetosyringone, an inducer of T-DNA transfer to plant cells. The results show that can transfer the Ti plasmid to with the same efficiency as within its own species. Conjugational transmission of extrachromosomal DNA like the narrow-host-range Ti plasmid may often not only occur among partners allowing propagation of the plasmid, but also on a ‘try-all’ basis including hosts which do not replicate the transferred DNA.


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