Functional characterization of the oligopeptide transporter operon oppABCDF-1 in Escherichia coli Nissle 1917.

BACKGROUND: The ABC-type oligopeptide transporter (Opp system), composed of five subunits (OppA-F), plays an essential role in microbial fitness and survival by mediating the uptake of short peptides. Paradoxically, although the probiotic strain Escherichia coli Nissle 1917 (EcN) harbors multiple Opp systems, it exhibits a defect in the import of oligopeptides comprising three or more amino acids, a limitation that may compromise its ability to colonize the intestinal tract. This functional paradox underscores the need for further investigation into the substrate specificity and physiological roles of Opp systems in EcN.

RESULTS: Genomic analysis identified four putative Opp systems in EcN, which exhibited low mutual homology (< 41% amino acid identity across all OppA-OppF components), indicating potential functional divergence. Deletion or overexpression of one of the operons, oppABCDF-1 (ECOLIN_07295-07315) did not alter EcN growth under standard or oligopeptide-supplemented conditions. Untargeted metabolomics further demonstrated that this operon was dispensable for oligopeptide uptake. Notably, metabolic profiling revealed significantly impaired uptake of three metabolites in the oppABCDF-1 deletion mutant, along with a modest reduction in the export of putrescine, austinoneol, and leucinic acid. These findings suggest that this Opp system may function as a bidirectional transporter with differential import and export activities.

CONCLUSION: These findings indicate that the oppABCDF-1 operon encoded by ECOLIN_07295-07315 in EcN is dispensable for oligopeptide assimilation, but may contribute to the transport of specific metabolites, offering new insights into the functional diversification of bacterial Opp systems.