Functional Redundancy in Perchlorate and Nitrate Electron Transport Chains and Rewiring Respiratory Pathways to Alter Terminal Electron Acceptor Preference.

Ouwei Wang,Hans K Carlson,Ryan A Melnyk,Misha G Mehta-Kolte,Matthew D Youngblut,John D Coates

doi:10.3389/fmicb.2018.00376

Abstract

Most dissimilatory perchlorate reducing bacteria (DPRB) are also capable of respiratory nitrate reduction, and preferentially utilize nitrate over perchlorate as a terminal electron acceptor. The similar domain architectures and phylogenetic relatedness of the nitrate and perchlorate respiratory complexes suggests a common evolutionary history and a potential for functionally redundant electron carriers. In this study, we identify key genetic redundancies in the electron transfer pathways from the quinone pool(s) to the terminal nitrate and perchlorate reductases in Azospira suillum PS (hereafter referred to as PS). We show that the putative quinol dehydrogenases, (PcrQ and NapC) and the soluble cytochrome electron carriers (PcrO and NapO) are functionally redundant under anaerobic growth conditions. We demonstrate that, when grown diauxically with both nitrate and perchlorate, the endogenous expression of NapC and NapO during the nitrate reduction phase was sufficient to completely erase any growth defect in the perchlorate reduction phase caused by deletion of pcrQ and/or pcrO. We leveraged our understanding of these genetic redundancies to make PS mutants with altered electron acceptor preferences. Deletion of the periplasmic nitrate reductase catalytic subunit, napA, led to preferential utilization of perchlorate even in the presence of equimolar nitrate, and deletion of the electron carrier proteins napQ and napO, resulted in concurrent reduction of nitrate and perchlorate. Our results demonstrate that nitrate and perchlorate respiratory pathways in PS share key functionally redundant electron transfer proteins and that mutagenesis of these proteins can be utilized as a strategy to alter the preferential usage of nitrate over perchlorate.

Highlights

Perchlorate (ClO−4 ) is deposited in the environment by both industrial activities and natural processes (Motzer, 2001; Urbansky, 2002; Dasgupta et al, 2005; Rajagopalan et al, 2009; Catling et al, 2010; Nilsson et al, 2013)
The pcrQ and pcrO genes are highly conserved among phylogenetically diverse dissimilatory perchlorate reducing bacteria (DPRB), suggesting that, as in PS, they are important for perchlorate metabolism
PcrQ is 79% identical to the PS NapC homolog (Potter and Cole, 1999). Both PcrQ and NapC are tetraheme c-type cytochromes belonging to the NapC/NirT family (Potter and Cole, 1999), and NapC is known to function as a quinol dehydrogenase that mediates electron transfer between quinone pool and soluble periplasmic nitrate reductase (Roldán et al, 1998; Potter and Cole, 1999; Brondijk et al, 2002)

Summary

Introduction

Perchlorate (ClO−4 ) is deposited in the environment by both industrial activities and natural processes (Motzer, 2001; Urbansky, 2002; Dasgupta et al, 2005; Rajagopalan et al, 2009; Catling et al, 2010; Nilsson et al, 2013). All isolated Gram-negative DPRM are facultative anaerobes, and the majority are capable of alternatively using nitrate as an electron acceptor (Herman and Frankenberger, 1999; Achenbach et al, 2001; Coates and Achenbach, 2004; Carlström et al, 2013, 2015) When both perchlorate and nitrate are present, pure, or mixed culture DPRM either preferentially or concurrently reduce nitrate, even though perchlorate respiration is energetically more favorable (E◦′ = +797 mV for the reduction of ClO−4 to Cl−, E◦′ = +750 mV for the reduction of NO−3 to N2) (Chaudhuri et al, 2002; Coates and Achenbach, 2004; Xiao and Roberts, 2013). New strategies to influence this electron acceptor utilization preference could improve treatment efficiency especially in environments where nitrate concentrations dominate (Youngblut et al, 2016b)

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Conclusion