Novel Metal Cation Resistance Systems from Mutant Fitness Analysis of Denitrifying Pseudomonas stutzeri
Authors: Vaccaro Brian J. , Lancaster W. Andrew , Thorgersen Michael P. , Zane Grant M. , Younkin Adam D. , Kazakov Alexey E. , Wetmore Kelly M. , Deutschbauer Adam , Arkin Adam P. , Novichkov Pavel S. , Wall Judy D. , Adams Michael W. W. 
Published: July 2016

Metal ion transport systems have been studied extensively, but the specificity of a given transporter is often unclear from amino acid sequence data alone. In this study, predicted Cu
2+
and Zn
2+
resistance systems in
Pseudomonas stutzeri
strain RCH2 are compared with those experimentally implicated in Cu
2+
and Zn
2+
resistance, as determined by using a DNA-barcoded transposon mutant library. Mutant fitness data obtained under denitrifying conditions are combined with regulon predictions to yield a much more comprehensive picture of Cu
2+
and Zn
2+
resistance in strain RCH2. The results not only considerably expand what is known about well-established metal ion exporters (CzcCBA, CzcD, and CusCBA) and their accessory proteins (CzcI and CusF), they also reveal that isolates with mutations in some predicted Cu
2+
resistance systems do not show decreased fitness relative to the wild type when exposed to Cu
2+
. In addition, new genes are identified that have no known connection to Zn
2+
(
corB
,
corC
, Psest_3226, Psest_3322, and Psest_0618) or Cu
2+
resistance (Mrp antiporter subunit gene, Psest_2850, and Psest_0584) but are crucial for resistance to these metal cations. Growth of individual deletion mutants lacking
corB
,
corC
, Psest_3226, or Psest_3322 confirmed the observed Zn-dependent phenotypes. Notably, to our knowledge, this is the first time a bacterial homolog of TMEM165, a human gene responsible for a congenital glycosylation disorder, has been deleted and the resulting strain characterized. Finally, the fitness values indicate Cu
2+
- and Zn
2+
-based inhibition of nitrite reductase and interference with molybdenum cofactor biosynthesis for nitrate reductase. These results extend the current understanding of Cu
2+
and Zn
2+
efflux and resistance and their effects on denitrifying metabolism.

IMPORTANCE
In this study, genome-wide mutant fitness data in
P. stutzeri
RCH2 combined with regulon predictions identify several proteins of unknown function that are involved in resisting zinc and copper toxicity. For zinc, these include a member of the UPF0016 protein family that was previously implicated in Ca
2+
/H
+
antiport and a human congenital glycosylation disorder, CorB and CorC, which were previously linked to Mg
2+
transport, and Psest_3322 and Psest_0618, two proteins with no characterized homologs. Experiments using mutants lacking Psest_3226, Psest_3322,
corB
,
corC
, or
czcI
verified their proposed functions, which will enable future studies of these little-characterized zinc resistance determinants. Likewise, Psest_2850, annotated as an ion antiporter subunit, and the conserved hypothetical protein Psest_0584 are implicated in copper resistance. Physiological connections between previous studies and phenotypes presented here are discussed. Functional and mechanistic understanding of transport proteins improves the understanding of systems in which members of the same protein family, including those in humans, can have different functions.