The MxtR/ErdR two-component system of Pseudomonas and Vibrio species has been reported to control the expression of genes required for acetate utilization [1,2]. A characteristic feature of the sensor kinase MxtR is the presence of a membrane-integrated solute/sodium symporter (SSS) domain connected to domains typical of bacterial sensor kinases via a STAC domain [3]. It is hypothesized that the SSS domain senses a yet unknown chemical signal, possibly transports the corresponding solute and influences the signal transduction process by associated conformational changes. The aims of the present work are (1) to elucidate possible additional metabolic processes controlled by MxtR/ErdR and (2) to determine the functional significance of individual domains of MxtR.

To address Aim 1, the growth of an mxtR mutant of the soil bacterium Pseudomonas putida KT2440 on different C-sources was investigated. Furthermore, electrophoretic mobility shift assays (EMSA) were performed to test the binding of the response regulator ErdR to promoters of potential target genes. To address Aim 2, domains of the sensor kinase MxtR were individually deleted and the growth of the corresponding mutants on different C- sources was examined.

Our results show that MxtR/ErdR is essential for growth on propionate as the sole carbon source in addition to acetate. EMSAs confirm that ErdR controls the expression of genes encoding enzymes involved in the activation (acyl-CoA synthesis) and degradation (methylcitrate cycle) of propionate. Furthermore, we observed that in addition to the histidine kinase (HisKA) domain, the SSS and STAC domains are essential for MxtR function. In contrast, the receiver (REC) domain of MxtR is dispensable for growth on short chain fatty acids.

The results obtained highlight the role of MxtR/ErdR in the regulation of short-chain fatty acid metabolism and demonstrate that the transporter domain of MxtR is essential for signal transduction in P. putida KT2440.

[1] Hang, S. et al. (2014) Cell host & microbe 16, 592-604.

[2] Henriquez, T. et al. (2023) Microbiol Spectr 11, e0292322.

[3] Korycinski, M. et al. (2025) J Mol Biol 427, 3327-3339