Acinetobacter calcoaceticus PHEA-2 utilizes phenol as its sole carbon and energy source and has a multi-component phenol hydroxylase-encoding gene operon (mphKLMNOP) for phenol degradation. Two additional genes, mphR and mphX, were found upstream and downstream of mphKLMNOP, respectively. The mphR gene encodes a XylR/DmpR-type regulator-like protein and is transcribed in the opposite direction to mphKLMNOP. The mphX gene is transcribed in the same direction as mphKLMNOP and encodes a protein with 293 amino acid residues showing weak identity with some unknown proteins encoded in the meta-cleavage pathway gene clusters for aromatic compound degradation. Disruption of mphR by homologous recombination resulted in the loss of phenol degradation while disruption of mphX caused significantly faster phenol degradation than in the wild type strain. Transcriptional assays for mphK, mphR, and mphX revealed that mphR activated mphKLMNOP transcription in the presence of phenol, but mphX partially repressed this activation. Gel mobility-shift assay demonstrated a direct interaction of MphR with the mphK promoter region. These results indicate the involvement of a novel repressor protein MphX in transcriptional regulation of phenol hydroxylase genes caused by a XylR/DmpR-type regulator MphR.

The gut microbiota plays a pivotal role in host health. Dietary components such as waxy maize starch (WMS) and malate-debranched WMS (MADBS) may serve as modulators of microbial composition and function. In this study, the effects of WMS and MADBS on murine gut microbiota in vivo and human fecal in vitro fermentation were investigated. The results of gut microbiota in mice revealed that WMS increased the abundance of Muribaculaceae and Bifidobacterium, while MADBS enriched Ileibacterium, Muribaculaceae, and Dubosiella. The in vitro fermentation model demonstrated that WMS increased the abundance of Bifidobacterium, Lactobacillus, Megamonas, and Megasphaera, whereas MADBS enhanced Weissella, Lactobacillus, and Prevotella. Both compounds decreased the levels of Escherichia-Shigella. Metabolically, compared to the control group, WMS improved the production of acetic, propionic, butyric, and valproic acids, while MADBS decreased the concentrations of all short-chain fatty acids (SCFAs). Compared to the control group, WMS reduced the production of CH₄, NH₃, and H₂S while increasing CO₂ yield. MADBS reduced the generation of CH₄, NH₃, H₂S, and CO₂. These findings suggest that WMS and MADBS can modulate the gut ecosystem by selectively promoting probiotics, inhibiting pathogens, and altering metabolic profiles.