MxtR/ErdR is a central regulator of short-chain fatty acid metabolism in Pseudomonas alloputida

Short-chain fatty acids (SCFAs) such as acetate and propionate represent important carbon and energy sources for environmental pseudomonads, requiring coordinated regulation to ensure efficient assimilation while maintaining metabolic homeostasis. In Pseudomonas alloputida KT2440, the two-component system MxtR/ErdR (CrbS/CrbR) is known to activate acetate assimilation through regulation of acsA-I. However, the full extent of the MxtR/ErdR regulon and its physiological role beyond acetate metabolism have remained unclear. We demonstrate that MxtR/ErdR directly regulates the methylcitrate cycle and coordinates both acetate and propionate metabolism. To define the MxtR/ErdR regulatory network, we combined comparative transcriptomics, physiological analyses, promoter-reporter assays, electrophoretic mobility shift assays (EMSAs) and targeted mutagenesis. Comparative transcriptomic analyses of the mxtR-H806N and {Delta}erdR mutants revealed extensive changes in gene expression, including coordinated downregulation of genes involved in propionate metabolism alongside genes associated with central carbon metabolism, transport, chemotaxis, and signal transduction. Consistent with these transcriptional changes, deletion of either mxtR or erdR abolished growth on propionate. Promoter-reporter assays and EMSAs demonstrated direct binding of ErdR to a conserved imperfect inverted repeat upstream of the prp gene cluster and prpE. Mutational analyses confirmed the functional importance of this binding motif for promoter activation. In addition, MxtR/ErdR contributed to pyruvate utilization through regulation of transport-associated genes, whereas deletion of downstream target genes only caused modest phenotypes, indicating that the physiological effects of MxtR/ErdR arise from coordinated regulation of multiple pathways. Our findings substantially expand the MxtR/ErdR regulon and identify this signaling system as a central regulator of SCFA homeostasis in P. alloputida KT2440.