Two-component systems (TCS) allow bacteria to sense and adapt to various extracellular stimuli by triggering cellular responses in the form of altered gene expression, inducing metabolic changes. The TCS MxtR/ErdR of Pseudomonas and Vibrio species has been shown to control the expression of genes essential to acetate utilisation [1,2]. MxtR contains a solute/sodium symporter (SSS) domain and transporters of the SSS family are known to be involved in the acquisition of solutes as carbon sources [3,4]. MxtR's SSS domain senses an unknown signal, transports the solute into the cell, affecting signal transduction and expression of target genes. We aim to (1) unravel the regulatory network of MxtR/ErdR and (2) identify the functional roles and significance of individual MxtR domains.

Pursuing Aim 1, the expression of anticipated target genes of the TCS was analysed in mxtR/erdR mutants. Thus, target genes were deleted and growth was investigated on different carbon sources. Moreover, binding between the response regulator ErdR and promoters of target genes was examined. Furthermore, we investigated the possibility of MxtR being a part of a sensor kinase network. To address Aim 2, domains of MxtR were deleted and growth on different carbon sources was analysed. Single amino acid substitutions in the SSS and receiver (REC) domain of MxtR, as well as the REC domain of ErdR were performed to evaluate their importance in signal transduction and carbon source utilisation.

Our results show MxtR/ErdR dependent expression of target genes encoding enzymes involved in the methylcitrate cycle i.e. the degradation of propionate. Deletion of the target genes reveal a role in propionate utilisation. Moreover, binding assays underline ErdR-target gene interaction and binding. In addition, we observed, that the SSS and STAC domains of MxtR are essential to its function and growth on various carbon sources, while the REC and PAS domains appear to be dispensable. We also identified amino acids in the SSS domain that are crucial to MxtR function and are hypothesized to play a key role in signal transduction.

The abovementioned results emphasize the role of MxtR/ErdR, its domains and target genes in short-chain fatty acid metabolism.

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

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

[3] Henriquez, T. et al. (2021) Int J Mol Sci 22,1880

[4] Jung, H. (2001) Biochim Biophys Acta 1505,131-143