Designating ligand specificities to metal uptake ABC transporters in Thermus thermophilus HB8

Abstract

Micronutrients such as metal ions are indispensable for the growth and survival of microorganisms in assorted environmental niches. However, change in cellular concentration of metal ions is pernicious for an organism; thus metal ion homeostasis is crucial for their survival and growth. An eminent mechanism for maintaining metal ion homeostasis in microorganisms is ATP-binding cassette (ABC) transporters, which transport metal ions in their ionic/complex forms across the cell membrane. For the uptake, metals are sequestered by substrate-binding proteins (SBPs) and transferred to transmembrane domains (TMDs) for their transport. In this work, a high-throughput data mining analysis has been performed to identify open reading frames (ORFs) encoding metal-specific ABC transporters in a thermophilic bacterium Thermus thermophilus HB8. In total, 22 ORFs resulting in eight ABC transport systems were identified, which are potentially involved in the uptake of metal ions. This study suggests that three out of eight metal-specific ABC import systems are specific to iron ions. Among the remaining five, two are particular to divalent metal ions such as Mg²⁺ and Zn²⁺/Mn²⁺, another two are for tetrahedral oxyanions such as MoO₄²⁻ and WO₄²⁻ and the remaining one imports cyanocobalamin (vitamin B12). Besides these, the results of this study demonstrate the existence of a mechanism where TMD and NBD components are shared among different ABC transport systems hinting that multiple substrates can be imported via a single transporter. This study thus provides the first ever preliminary glimpse into the entire repertoire of metal uptake ABC transporters in a thermophilic organism.