"Methylotuvimicrobium buryatense" 5GB1C lanthanum-response repressor mutants

Several of the metabolic enzymes in methanotrophic bacteria rely on metals for both their expression and their catalysis. The MxaFI methanol dehydrogenase enzyme complex uses calcium as a cofactor to oxidize methanol, while the alternative methanol dehydrogenase XoxF uses lanthanide metals such as lanthanum and cerium for the same function. Lanthanide metals, abundant in the earth’s crust and widely used in electronic devices, strongly repress the transcription of mxaF yet activate the transcription of xoxF. This regulatory program, called the “lanthanide switch,” is central to methylotrophic metabolism, but its components are unknown. To uncover additional components of the lanthanide switch, we designed a mutagenesis system in the Type I gammaproteobacterial methanotroph “Methylotuvimicrobium buryatense” 5GB1C and selected for mutants unable to repress the mxaF promoter in the presence of lanthanum. Whole genome resequencing for multiple lanthanide switch mutants identified several unique point mutations in a single gene encoding a TonB-dependent receptor, which we have named LanA. While the LanA TonB-dependent receptor is absolutely required for the lanthanide switch, it does not affect lanthanum uptake by the bacterium. Deletion of a separate lanthanide-repressible gene encoding a TonB protein had no effect on the lanthanide switch or on lanthanum uptake. The discovery of this novel component of the lanthanide regulatory system highlights the complexity of this circuit and suggests further components are likely involved.