During growth on the C substrates methanol or methylamine, 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, (formerly known as ) and a third gene (formerly known as ) 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 and . Expression of the structural genes was analysed by measuring the expression of a ()- transcriptional fusion in the presence of or combinations of these genes. Mutants that were unable to express were impaired for growth on methanol, did not synthesize MDH and could not express a - transcriptional fusion. This indicates that the response regulator MxaX is essential for expression of the structural genes. Mutants that had a deletion in or both and were able to grow on methanol and were able to regulate the expression of the - fusion just like the wild-type. These findings indicate that and 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.


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