Efficient vir gene induction in Agrobacterium tumefaciens requires virA, virG, and vir box from the same Ti plasmid

The vir genes of octopine, nopaline, and L,L-succinamopine Ti plasmids exhi bit structural and functional similarities. However, we observed difference s in the interactions between octopine and nopaline vir components. The ind uction of an octopine virE(A6)::lacZ fusion (pSM358cd) was 2.3-fold higher in an octopine strain (A348) than in a nopaline strain (C58). Supplementati on of the octopine virG(A6) in a nopaline strain with pSM358 did not comple tely restore virE(A6) induction. However, addition of the octopine virA(A6) to the above strain increased virE(A6) induction to a level almost compara ble to that in octopine strains. In a reciprocal analysis, the induction of a nopaline virE(C58)::cat fusion (pUCD1553) was two- to threefold higher i n nopaline (C58 and T37) strains than in octopine (A348 and AchS) and L,L-s uccinamopine (A281) strains. Supplementation of nopaline virA(C58) and virG (C58) in an octopine strain (A348) harboring pUCD1553 increased induction l evels of virEC58::cat fusion to a level comparable to that in a nopaline st rain (C58). Our results suggest that octopine and L,L-succinamopine VirG pr oteins induce the octopine virE(A6) more efficiently than they do the nopal ine virE(C58). Conversely, the nopaline VirG protein induces the nopaline v irE(C58) more efficiently than it does the octopine virE(A6). The ability o f Bo542 virG to bring about supervirulence in tobacco is observed for an oc topine vir helper (LBA4404) but not for a nopaline vir helper (PMP90). Our analyses reveal that quantitative differences exist in the interactions bet ween VirG and vir boxes of different Ti plasmids, Efficient vir gene induct ion in octopine and nopaline strains requires virA, virG, and vir boxes fro m the respective Ti plasmids.