A DIFFUSIBLE COMPOUND CAN ENHANCE CONJUGAL TRANSFER OF THE TI PLASMID IN AGROBACTERIUM-TUMEFACIENS

Several octopine strains of Agrobacterium tumefaciens were tested for Ti plasmid (pTi) transfer after induction by 400-mu-g of octopine per ml for 24 h. The strains could be divided into two groups, transfer efficient (Tra(e)) and transfer inefficient (Tra(ie)); the respective rates of transfer were 0.77 x 10(-2) to 1.14 x 10(-2) and 0.33 x 10(-6) to 9.8 x 10(-6) plasmid transconjugant per donor cell. Transfer efficiencies of Tra(ie) strains were greatly increased when the time of induction was 72 h. A diffusible conjugation factor (CF) that can enhance conjugal transfer of pTi in A. tumefaciens was discovered when both Tra(e) and Tra(ie) donor strains were induced in the same plate. The evidence indicates that CF is a key factor affecting transfer efficiency of pTi but is not sufficient by itself to induce transfer. Tra(c) mutants can produce CF constitutively, and Tra(e) strains can produce it after induction by low octopine concentrations. The transfer efficiency of Tra(ie) strains was greatly increased by adding CF to the induction medium. The thermosensitive strain B6S3, which normally cannot conjugate at temperatures above 30-degrees-C, could transfer pTi efficiently at 32 and 34-degrees-C in the presence of CF. Production of CF is dependent on the presence of pTi but appears to be common for different opine strains; it was first detected in octopine strains, but nopaline strains also produced the same or a similar compound. CF is very biologically active, affecting donor but not recipient bacterial cells, but CF does not promote aggregation. Data suggest that CF might be an activator or derepressor in the conjugation system of A. tumefaciens. CF is a dialyzable small molecule and is resistant to DNase, RNase, protease, and heating to 100-degrees-C for 10 min, but autoclaving (121-degrees-C for 15 min) and alkaline treatment removed all activity.