Identification and characterization of an indigo-producing oxygenase involved in indole 3-acetic acid utilization by Acinetobacter baumannii.

Acinetobacter baumannii harbours a gene cluster similar to the iac locus of Pseudomonas putida 1290, which can catabolize the plant hormone indole 3-acetic acid (IAA) as an energy source. However, there has been no evidence showing that IAA can be utilized by A. baumannii. This study showed that A. baumannii can grow in M9 minimal medium containing IAA as the sole carbon source. A mutagenesis study indicated that iacA, encoded in the iac locus of A. baumannii, is involved in the catabolism of IAA. As shown by western blotting analysis, the IacA protein was detected in A. baumannii grown in M9 minimal medium with IAA but not with pyruvate, suggesting that the expression of iacA is regulated by the presence of IAA. In vitro studies have shown that IacA can oxidize indole, an IAA-like molecule, converting it to indoxyl, which spontaneously dimerises to form indigo. In this study, we show that the crude extracts from either wild-type A. baumannii or Escherichia coli overexpressing IacA can oxidize IAA. These results imply that the iac gene cluster of A. baumannii is involved in IAA degradation and that the iacA gene is upregulated when cells encounter IAA in their native environments.

The S subunit of D-ornithine aminomutase from Clostridium sticklandii is responsible for the allosteric regulation in D-alpha-lysine aminomutase.

Ornithine and lysine are degraded in quite a similar way in Clostridium sticklandii. Both pathways involve adenosylcobalamin-dependent enzymes, d-ornithine 4,5-aminomutase and lysine 5,6-aminomutase. According to previous reports, lysine 5,6-aminomutase is an ATP-dependent allosteric enzyme with many different activators and inhibitors. However, recent studies indicate that ATP does not have a regulatory effect on the recombinant enzyme. To monitor the activity of lysine aminomutase, a novel capillary electrophoresis-based assay method was developed. The present results demonstrate that the S subunit of d-ornithine aminomutase, OraS, is capable of forming a complex with KamDE of lysine 5,6-aminomutase and restores the enzyme's ATP-dependent allosteric regulation. Not only does ATP lower the K(m) of the KamDE-OraS complex for adenosylcobalamin and pyridoxal phosphate, but also OraS protein alone lowers the K(m) of KamDE for adenosylcobalamin and pyridoxal phosphate. The activity of reconstituted enzyme can also be activated by ammonium ion as reported by Morley and Stadtman.