Characterization of transcription organization and analysis of unique expression patterns of an alkyl hydroperoxide reductase C gene (ahpC) and the peroxide regulator operon ahpF-oxyR-orfX from Xanthomonas campestris pv. phaseoli

Abstract

We have analyzed the transcription organization of ahpC, ahpF, oxyR, and orfX from Xanthomonas campestris pv. phaseoli. ahpC was transcribed as a monocistronic 0.6-kb mRNA, while ahpF-oxyR-orfX were transcribed as a polycistronic approximately 3.0-kb-long mRNA. The novel transcription organization of these genes has not observed in other bacteria. Western analysis showed that oxidants (peroxides and superoxide anions), a thiol reagent (N-ethylmaleimide), and CdCl 2 caused large increases in the steady-state level of AhpC. Growth at alkaline pH also moderately induced AhpC accumulation. Thermal and osmotic stresses did not alter the levels of AhpC. Northern blotting results confirmed that oxidant-and CdCl 2 -induced AhpC accumulation was due to increased levels of ahpC transcripts. Analysis of oxyR expression revealed a unique pattern. Unlike other bacterial systems, peroxides and a superoxide generator induced accumulation of OxyR. Northern blotting results confirmed that these oxidants induced expression of oxyR operon. This novel regulatory pattern could be generally important. The transcription organization and patterns of chemicals and stress induction of ahpC and oxyR differed from those of other bacteria and are likely to be important for X. campestris pv. phaseoli survival during exposure to oxidants.

The katA Catalase Gene Is Regulated by OxyR in both Free-Living and Symbiotic Sinorhizobium meliloti

“…The position of oxyR is very variable in the different bacterial genomes, but it is often next to a gene involved in oxidative stress protection and regulated by OxyR: for example, ahpC in Mycobacterium tuberculosis (7) and Xanthomonas campestris pv. phaseoli (25), dps in Bacteroides fragilis (32), oxyS in E. coli (4), recG in Pseudomonas aeruginosa (28), kat in B. abortus (17). Similar genomic relationships between oxyR and catalase genes are observed for most members of the Rhizobiales studied so far, such as S. meliloti, Agrobacterium tumefaciens, B. abortus, Mesorhizobium loti, Rhizobium etli, and Rhizobium leguminosarum bv.…”

Regulation of inducible peroxide stress responses

“…In vivo experiments from various bacteria show that high concentrations of superoxide generators (e.g. >0.3 mM paraquat) and S-nitrosothiols also induce the expression of OxyR-regulated genes (Mongkolsuk et al, 1997;Storz and Zheng, 2000;Zheng et al, 2001b). However, it is not clear whether these compounds oxidize OxyR directly or are acting indirectly by, for example, altering cellular thiol/disulphide homeostasis.…”

“…The C199 and C208 residues involved in H 2 O 2 sensing are absolutely conserved, suggesting a common redox-sensing mechanism . However, not all aspects of OxyR regulation are similar to E. coli: in Xanthomonas campestris, exposure to oxidant results in changes in both the level and activity of OxyR (Mongkolsuk et al, 1997).…”

The Catalase-Peroxidase KatG Is Required for Virulence of Xanthomonas campestris pv. campestris in a Host Plant by Providing Protection against Low Levels of H 2 O 2

“…Reduced and oxidized OxyR regulons bind differently to the target promoters, and both repress or induce transcription of the target genes (28). We have been investigating the OxyRmediated response to peroxide stress in Xanthomonas campestris (2,5,14,17,18). The expression of ahpCF and katA, which encode an alkyl hydroperoxide reductase and a monofunctional catalase, respectively, are governed by OxyR.…”