Defining the binding determinants of OxyR: Implications for the link between the contracted OxyR regulon and adaptation.

It is well-established that OxyR functions as a transcriptional activator of the peroxide stress response in bacteria, primarily based on studies on Recent investigations have revealed that OxyRs of some other bacteria can regulate gene expression through both repression and activation or repression only; however, the underlying mechanisms remain largely unknown. Here, we demonstrated in γ-proteobacterium regulation of OxyR on expression of major catalase gene in a dual-control manner through interaction with a single site in the promoter region. Under non-stress conditions, expression was repressed by reduced OxyR (OxyR), whereas when oxidized, OxyR (OxyR) outcompeted OxyR for the site because of substantially enhanced affinity, resulting in a graded response to oxidative stress, from repression to derepression to activation. The OxyR-binding motif is characterized as a combination of the motif (tetranucleotides spaced by heptanucleotide) and palindromic structure. We provided evidence to suggest that the OxyR regulon is significantly contracted compared with those reported, probably containing only five members that are exclusively involved in oxygen reactive species scavenging and iron sequestering. These characteristics probably reflect the adapting strategy of the bacteria that represents to thrive in redox-stratified microaerobic and anaerobic environments.