Genetic redundancy in iron and manganese transport in the metabolically versatile bacterium Rhodopseudomonas palustris TIE-1

Abstract

The purple non-sulfur bacterium Rhodopseudomonas palustris TIE-1 can produce useful biochemicals such as bioplastics and biobutanol. Production of such biochemicals requires intracellular electron availability, which is governed by the availability and the transport of essential metals such as iron (Fe). Because of the distinct chemical properties of ferrous [Fe(II)] and ferric iron [Fe(III)], different transport systems are required for their transport and storage in bacteria. Although Fe(III) transport systems are well characterized, we know much less about Fe(II) transport systems except for the FeoAB system. Iron transporters can also import manganese (Mn). Here, we study Fe and Mn transport by five putative Fe transporters in TIE-1 under metal-replete, -deplete, oxic and anoxic conditions. We observe that by overexpressing feoAB, efeU, and nramp1AB, the intracellular concentration of Fe and Mn can be enhanced in TIE-1, under oxic and anoxic conditions, respectively. The deletion of a single gene/operon does not attenuate Fe or Mn uptake in TIE-1 regardless of the growth conditions used. This indicates that genetically dissimilar yet functionally redundant Fe transporters in TIE-1 can complement each other. Relative gene expression analysis shows that feoAB and efeU are expressed during Fe and Mn depletion under both oxic and anoxic conditions. The promoters of these transporter genes contain a combination of Fur and Fnr boxes suggesting that their expression is regulated by both Fe and oxygen availability. The findings from this study will help us modulate intracellular Fe and Mn concentration, ultimately improving TIE-1s ability to produce desirable biomolecules. IMPORTANCERhodopseudomonas palustris TIE-1 is a metabolically versatile bacterium that can use various electron donors including Fe(II) and poised electrodes for photoautotrophic growth. TIE-1 can produce useful biomolecules such as biofuels and bioplastics during various growth conditions. Production of such reduced biomolecules is controlled by intracellular electron availability, which in turn is mediated by various iron-containing proteins in the cell. Several putative Fe transporters exist in TIE-1s genome. Some of these transporters can also transport Mn, part of several important cellular enzymes. Therefore, understanding the ability to transport and respond to varying levels of Fe and Mn under different conditions is important to improve TIE-1s ability to produce useful biomolecules. Our data suggest that by overexpressing Fe transporter genes via plasmid-based expression, we can increase the import of Fe and Mn in TIE-1. Future work will leverage these data to improve TIE-1 as an attractive microbial chassis and future biotechnological workhorse.