EXPRESSION OF THE STRUCTURAL MAX 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 C-1 substrates methanol or methylamine, Paracoccu s denitrificans is able to activate the expression of the genes encodi ng methanol dehydrogenase. In a previous paper the isolation of an ope ron containing two regulatory genes, mxaYX(formerly known as moxYX) an d a third gene mxaZ (formerly known as moxZ) was described. MxaY and M xaX were shown to have homology with the signal sensors and the respon se regulators, respectively. Here we describe the isolation and charac terization of mutants with marked and unmarked mutations in mxaZ, mxaY and mxaX., Expression of the structural mox genes was analysed by mea suring the expression of a mxaF(moxF)-lacZ transcriptional fusion in t he 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 transcripti onal fusion. This indicates that the response regulator MxaX is essent ial for expression of the structural mox genes. Mutants that had a del etion 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 n ot essential for normal C-1 regulation. In addition the results sugges t that, at least in the absence of the signal sensor MxaY, MxaX can be activated via a different, but parallel, signal transduction pathway.