Isolation and analysis of the Xanthomonas alkyl hydroperoxide reductase gene and the peroxide sensor regulator genes ahpC and ahpF-oxyR-orfX.

From Xanthomonas campestris pv. phaseoli, we have isolated by two independent methods genes involved in peroxide detoxification (ahpC and ahpF), a gene involved in peroxide sensing and transcription regulation (oxyR), and a gene of unknown function (orfX). Amino acid sequence analysis of AhpC, AhpF, and OxyR showed high identity with bacterial homologs. OrfX was a small cysteine-rich protein with no significant homology to known proteins. The genes ahpC, ahpF, oxyR, and orfX were arranged in a head-to-tail fashion. This unique arrangement was conserved in all of the Xanthomonas strains tested. The functionalities of both the ahpC and oxyR genes were demonstrated. In X. campestris pv. phaseoli, increased expression of ahpC alone conferred partial protection against growth retardation and killing by organic hydroperoxides but not by H2O2 or superoxide generators. These genes are likely to have important physiological roles in protection against peroxide toxicity in Xanthomonas.