phiXYZp

phiXYZp is a protein that was initially identified during a large-scale proteomic screening of the marine bacterium Pseudomonas sp. strain MC12. The gene encoding phiXYZp is located on a 12-kilobase plasmid that also carries several antibiotic resistance determinants. Bioinformatic analysis places the protein in the DUF1234 family of proteins, for which no definitive function has been assigned.

Structural prediction models suggest that phiXYZp adopts a Rossmann-like fold and contains a conserved glycine-rich loop that may indicate an NAD(P)+ binding motif. Experimental studies using recombinant protein expressed in Escherichia coli revealed that phiXYZp exhibits oxidoreductase activity toward a panel of small-molecule substrates, preferring 2-hydroxyacetophenone as a substrate with a k_cat of 3.2 s−1. Site‑directed mutagenesis of the predicted active-site residues, such as Lys107 and Asp135, abolished enzymatic activity, supporting the catalytic role of these residues.

In vitro assays show that phiXYZp expression correlates with increased tolerance to oxidative stress, suggesting a role in host defense against reactive oxygen species. Transcriptional profiling indicates that the gene is upregulated during exponential growth and under low-oxygen conditions, consistent with a response to metabolic stress.

Although the precise physiological substrate of phiXYZp in its native host remains uncertain, the protein has sparked interest as a potential target for antimicrobial development. Ongoing work aims to elucidate its crystal structure and explore its involvement in plasmid-mediated fitness traits.