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Description
Microbial utilization of single carbon (C1) compounds such as methane and methanol is known as methylotrophy. Key enzymes involved in methylotrophy require minerals, such as copper (Cu), iron (Fe), calcium (Ca) or nickel (Ni), as co-factors. The recent discovery of the alternative methanol dehydrogenase, XoxF, which uses lanthanide metals (Ln) as its co-factors, expands the list of metal-dependent C1-reactions. The fact that methylotrophs efficiently sequester metals from the environment leads to the hypothesis that XoxF- containing methylotrophs have specific cellular machineries for the scavenging, uptake, and utilization of lanthanides. Understanding these mechanisms could provide novel solutions for the extraction and recycling of lanthanides, which have become critical components in many high technological devices. In this study, I investigate mechanisms that contribute to copper and lanthanide sequestration in the obligate methanotrophic bacteria Methylotuvimicrobium alcaliphilum (20ZR) and Methylotuvimicrobium buryatense (5GB1). First, a set of cell fractionation studies was carried out to isolate the surface (S)-layer proteins (SLP) and their associated proteins, including outer membrane proteins (Omp), from bacteria grown in metal-rich and metal-limited conditions. After this, a proteomics study was performed in collaboration with the Pacific Northwest National Laboratory (Richland, WA) to determine the composition of the S-layer matrix, as well as compositional changes induced by metal scarcity. The study revealed that the expression of the SLP remains constant across different growth conditions, supporting previous electron microscopy (EM) observations. However, an increase in expression of surface associated putative metal binding and transport systems, and numerous TonB-receptors was observed in cells grown in metal-limited conditions. Subsequently, mutagenesis studies and physiological tests were carried out to observe the cellular phenotype associated with the lack of S-layer core proteins. The phenotype observed for slp-mutants resembled those observed for copper-repressible polypeptide, such as CorA. Furthermore, cells lacking the slp-genes show disparate responses toward different lanthanides. Results of these studies suggest that the S-layer proteins may act as an anchorage for metal-binding proteins that allow metal acquisition. This discovery highlights the possibility of utilizing methanotrophic S-layers as efficient scaffolding proteins for selective bioleaching of Ln from natural sources or electronic waste.
