mutant fitness in microoxia is supported by an Anr-regulated oxygen-binding hemerythrin.

strains with loss-of-function mutations in the transcription factor LasR are frequently encountered in the clinic and the environment. Among the characteristics common to LasR-defective (LasR-) strains is increased activity of the transcription factor Anr, relative to their LasR+ counterparts, in low-oxygen conditions. One of the Anr-regulated genes found to be highly induced in LasR- strains was (), which we named for microoxic hemerythrin. Purified Mhr protein contained the predicted di-iron center and bound molecular oxygen with an apparent of ∼1 µM. Both Anr and Mhr were necessary for fitness in + and mutant strains in colony biofilms grown in microoxic conditions, and the effects were more striking in the mutant. Among genes in the Anr regulon, was most closely coregulated with the Anr-controlled high-affinity cytochrome oxidase genes. In the absence of high-affinity cytochrome oxidases, deletion of no longer caused a fitness disadvantage, suggesting that Mhr works in concert with microoxic respiration. We demonstrate that Anr and Mhr contribute to LasR- strain fitness even in biofilms grown in normoxic conditions. Furthermore, metabolomics data indicate that, in a mutant, expression of Anr-regulated leads to differences in metabolism in cells grown on lysogeny broth or artificial sputum medium. We propose that increased Anr activity leads to higher levels of the oxygen-binding protein Mhr, which confers an advantage to mutants in microoxic conditions.