Fig. 5.

Expression of mxaF-yfp and xox1-yfp promoter fusions in M. extorquens AM1 strains. (A) mxa (black) and xox1 (white) promoter activities were determined by calculating the relative fluorescence units (RFU) per OD from 3 biological replicates grown to the mid-log phase (OD of ∼0.6) in medium containing both succinate and methanol. The background level from a promoterless yfp fusion vector was 3,940 ± 70 RFU/OD. (B) Current model for how MxcE and MxbM regulate expression of the mxa operon. In this model, both the MxcE and MxbM response regulators are required for the expression of the mxa operon, with MxcE having an indirect effect. MeDH represents production of the methanol dehydrogenase gene. MxcE has been shown to be involved in the activation of expression from the mxbDM promoter region, predicting a cascade of regulation, with MxcE activating the expression of mxbM, which, when translated, activates expression from the mxa promoter. The data in panel A added to the model shown passed the dashed line, suggesting that MxbM may act as a repressor of the xox1 operon. The direct binding of both MxcE and MxbM has not yet been demonstrated.