A single amino acid substitution beyond the C2H2-zinc finger in Ros derepresses virulence and T-DNA genes in Agrobacterium tumefaciens

Ros is a chromosomally-encoded repressor containing a novel C2H2 zinc finge r in Agrobacterium tumefaciens. Ros regulates the expression of six virulen ce genes and an oncogene on the Ti plasmid. Constitutive expression of thes e genes occurs in the spontaneous mutant 4011R derived from the octopine st rain Ach-5, resulting in T-DNA processing in the absence of induction, and in the biosynthesis of cytokinin. Interestingly, the mutation in 4011R is a n Arg to Cys conversion at amino acid residue 125 near the C-terminus well outside the zinc finger of Ros. Yet, Ros bearing this mutation is unable to bind to the Res-box and is unable to complement other ros mutants. (C) 200 0 Federation of European Microbiological Societies. Published by Elsevier S cience B.V. All rights reserved.