The chromosomal response regulatory gene chvI of Agrobacterium tumefaciens complements an Escherichia coli phoB mutation and is required for virulence.

In an effort to identify the Agrobacterium tumefaciens phosphate regulatory gene(s), we isolated a clone from an A. tumefaciens cosmid library that restored regulated alkaline phosphatase activity to an Escherichia coli phoB mutant. The gene that complemented phoB was localized by subcloning and deletion analysis, and the DNA sequence was determined. An open reading frame, denoted chvI, was identified that encoded a predicted protein with amino acid similarity to the family of bacterial response regulators and 35% identify to PhoB. Surprisingly, an A. tumefaciens chvI mutant showed normal induction of phosphatase activity and normal virG expression when grown in phosphate-limiting media. However, this mutant was unable to grow in media containing tryptone, peptone, or Casamino Acids and was also more sensitive than the wild type to acidic extracellular pH. This mutant was avirulent on Kalanchoeë diagremontiana and was severely attenuated in vir gene expression. The pH-inducible expression of virG was also abolished. Growth of the chvI mutant was inhibited by K. diagremontiana wound sap, suggesting that avirulence may be due, in part, to the inability of this mutant to survive the plant wound environment.