Identification and characterization of TrxA: a novel bacterial thioredoxin–metallothionein chimera

Heavy metals (Hms) are naturally occurring elements which can be essential, such as zinc, copper, and iron, or non-essential, including cadmium, mercury, and lead. While essential metals serve as cofactors in critical enzymatic processes, elevated concentrations of both essential and non-essential Hms pose severe toxicity risks, primarily through oxidative stress, disruption of metal homeostasis, and biomolecular damage. Microorganisms have evolved diverse mechanisms to cope with metal-induced stress, including metal sequestration, enzymatic transformation, efflux systems, and surface immobilization. Among these, metallothioneins (Mts) are small, cysteine-rich proteins capable of high-affinity metal binding, contributing to cellular detoxification. Although Mts have been extensively studied in eukaryotes, knowledge of bacterial Mts remains limited, with characterized examples largely confined to cyanobacteria and a few other bacterial species. In this study, we identified a novel hybrid protein, TrxA, from Runella aurantiaca, containing a thioredoxin (Trx) domain fused to a Mt domain. The presence of the Trx domain may confer improved stability and solubility, supporting potential recombinant applications. In fact, the recombinant protein, named TrxMt, was heterologously expressed in Escherichia coli, displaying both disulfide-reducing activity and heavy metal-binding capability. Notably, TrxMt expression, enhanced bacterial tolerance to multiple Hms, demonstrating its functional relevance in vivo. These findings expand the understanding of bacterial Mt diversity and suggest that TrxMt is a promising candidate for the bioremediation of heavy metal–contaminated environments, combining metal detoxification with favorable biochemical properties for industrial and environmental applications.