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

Abstract

The vir genes of octopine, nopaline, and L,L-succinamopine Ti plasmids exhibit structural and functional similarities. However, we observed differences in the interactions between octopine and nopaline vir components. The induction of an octopine virE_A6::lacZ fusion (pSM358cd) was 2.3-fold higher in an octopine strain (A348) than in a nopaline strain (C58). Supplementation of the octopine virG_A6 in a nopaline strain with pSM358 did not completely restore virE_A6 induction. However, addition of the octopine virA_A6 to the above strain increased virE_A6 induction to a level almost comparable to that in octopine strains. In a reciprocal analysis, the induction of a nopaline virE_C58::cat fusion (pUCD1553) was two- to threefold higher in nopaline (C58 and T37) strains than in octopine (A348 and Ach5) and L,L-succinamopine (A281) strains. Supplementation of nopaline virA_C58 and virG_C58 in an octopine strain (A348) harboring pUCD1553 increased induction levels of virE_C58::cat fusion to a level comparable to that in a nopaline strain (C58). Our results suggest that octopine and L,L-succinamopine VirG proteins induce the octopine virE_A6 more efficiently than they do the nopaline virE_C58. Conversely, the nopaline VirG protein induces the nopaline virE_C58 more efficiently than it does the octopine virE_A6. The ability of Bo542 virG to bring about supervirulence in tobacco is observed for an octopine vir helper (LBA4404) but not for a nopaline vir helper (PMP90). Our analyses reveal that quantitative differences exist in the interactions between VirG and vir boxes of different Ti plasmids. Efficient vir gene induction in octopine and nopaline strains requires virA, virG, and vir boxes from the respective Ti plasmids.