VARIABLE EFFICIENCY OF A TI PLASMID-ENCODED VIRA PROTEIN IN DIFFERENTAGROBACTERIAL HOSTS

The transconjugant CB100, harboring the Ti plasmid from the Agrobacter ium tumefaciens biovar 2 strain D10B/87 in the chromosomal background of the biovar 1 strain C58, was defective in vir gene induction. This defect was corrected in the presence of virA from pTiA6. Based on this complementation result and an analysis of the induction requirements of the transconjugant CB100 and its parent strains, it was hypothesize d that the defective vir gene induction in CB100 was related to a dysf unctional interaction between the pTi-encoded D10B/87 VirA and the chr omosome-encoded C58 ChvE. To verify this hypothesis, D10B/87 and C58 v irA were compared, and conclusions from this first set of analyses wer e then corroborated by comparing D10B/87 and C58 chvE. Whereas only a few nucleotide differences were identified in the promoters and 5' end s of the coding regions of D10B/87 and C58 virA, analysis of hybrid vi rA genes showed that these differences collectively accounted for the poor vir gene induction of strain CB100. In contrast with the sequence similarity of the VirA proteins, extensive divergence was seen betwee n the chromosome-encoded D10B/87 and C58 ChvE. Although D10B/87 chvE i ntroduced in trans had little effect on vir gene induction of CB100, i t enhanced the induction response of a strain CB100. derivative in whi ch the chromosomal C58 chvE had been inactivated by marker exchange. T hese results suggest that chromosomal backgrounds provided by differen t strains of A. tumefaciens are not equivalent for VirA function. Foll owing conjugative transfer of certain Ti plasmids to a new agrobacteri al host, evolution of the newly introduced virA, or coevolution of chv E and virA, may lead to optimization of ChvE-VirA interaction and vir gene induction levels.