Expression of the structural mox genes in Paracoccus denitrificans follows wild-type regulation in mutants with a deletion in mxaY, the gene encoding the signal sensor

During growth on the C₁ substrates methanol or methylamine, Paracoccus denitrificans is able to activate the expression of the genes encoding methanol dehydrogenase. In a previous paper the isolation of an operon containing two regulatory genes, mxaYX (formerly known as moxYX) and a third gene mxaZ (formerly known as moxZ) was described. MxaY and MxaX were shown to have homology with the signal sensors and the response regulators, respectively. Here we describe the isolation and characterization of mutants with marked and unmarked mutations in mxaZ mxaY and mxaX. Expression of the structural mox genes was analysed by measuring the expression of a mxaF(moxF)-lacZ transcriptional fusion in the presence of mxaZ, mxaY, mxaX or combinations of these genes. Mutants that were unable to express mxaX were impaired for growth on methanol, did not synthesize MDH and could not express a mxaF-lacZ transcriptional fusion. This indicates that the response regulator MxaX is essential for expression of the structural mox genes. Mutants that had a deletion in mxaY or both mxaY and mxaZ were able to grow on methanol and were able to regulate the expression of the mxaF-lacZ fusion just like the wild-type. These findings indicate that mxaY⁺ and mxaZ⁺ are not essential for normal C, regulation. In addition the results suggest that, at least in the absence of the signal sensor MxaY, MxaX can be activated via a different, but parallel, signal transduction pathway.