Unusual Relationship between Iron Deprivation and Organophosphate Hydrolase Expression.

Organophosphate hydrolases (OPH), hitherto known to hydrolyze the third ester bond of organophosphate (OP) insecticides and nerve agents, have recently been shown to interact with outer membrane transport components, namely, TonB and ExbB/ExbD. In an OPH negative background, Sphingopyxis wildii cells failed to transport ferric enterobactin and showed retarded growth under iron-limiting conditions. We now show the OPH-encoding organophosphate degradation (opd) gene from Sphingobium fuliginis ATCC 27551 to be part of the iron regulon. A fur-box motif found to be overlapping with the transcription start site (TSS) of the opd gene coordinates with an iron responsive element (IRE) RNA motif identified in the 5' coding region of the opd mRNA to tightly regulate opd gene expression. The fur-box motif serves as a target for the Fur repressor in the presence of iron. A decrease in iron concentration leads to the derepression of opd. IRE RNA inhibits the translation of opd mRNA and serves as a target for apo-aconitase (IRP). The IRP recruited by the IRE RNA abrogates IRE-mediated translational inhibition. Our findings establish a novel, multilayered, iron-responsive regulation that is crucial for OPH function in the transport of siderophore-mediated iron uptake. IMPORTANCE Sphingobium fuliginis, a soil-dwelling microbe isolated from agricultural soils, was shown to degrade a variety of insecticides and pesticides. These synthetic chemicals function as potent neurotoxins, and they belong to a class of chemicals termed organophosphates. S. fuliginis codes for OPH, an enzyme that has been shown to be involved in the metabolism of several organophosphates and their derivatives. Interestingly, OPH has also been shown to facilitate siderophore-mediated iron uptake in S. fuliginis and in another Sphingomonad, namely, Sphingopyxis wildii, implying that this organophosphate-metabolizing protein has a role in iron homeostasis, as well. Our research dissects the underlying molecular mechanisms linking iron to the expression of OPH, prompting a reconsideration of the role of OPH in Sphingomonads and a reevaluation of the evolutionary origins of the OPH proteins from soil bacteria.

Proteomics of Sphingobium indicum B90A for a deeper understanding of hexachlorocyclohexane (HCH) bioremediation.

Genome wide expression profiling of Sphingobium indicum B90A revealed induction of lin genes, linA and linB, involved in dechlorination of hexachlorocyclohexane (HCH), in the presence of all four isomers of HCH. Supporting proteomics data, the qPCR and promoter assay showed upregulation of linA transcription in the presence of HCH isomers. Analysis of the upstream region of the linA gene revealed the existence of the GntR binding site overlapping the -10 hexamer of the putative promoter motif. As GntR is a known transcription repressor its dissociation from the linA promoter is expected to induce lin genes in the presence of HCH isomers. Comparison of in situ and in-culture proteomics indicated expression lin genes at the dumpsite, an indication for the in situ HCH degradation.