The ability to conserve energy in the presence or absence of oxygen provides a
metabolic versatility that confers an advantage in natural ecosystems. The
switch between alternative electron transport systems is controlled by the
fumarate nitrate reduction transcription factor (FNR) that senses oxygen via an
oxygen-sensitive [4Fe-4S]2+ iron-sulfur cluster. Under O2 limiting conditions,
FNR plays a key role in allowing bacteria to transition from aerobic to
anaerobic lifestyles. This is thought to occur via transcriptional activation of
genes involved in anaerobic respiratory pathways and by repression of genes
involved in aerobic energy production. The Proteobacterium Acidithiobacillus
ferrooxidans is a model species for extremely acidophilic microorganisms that
are capable of aerobic and anaerobic growth on elemental sulfur coupled to
oxygen and ferric iron reduction, respectively. In this study, an FNR-like
protein (FNRAF) was discovered in At. ferrooxidans that exhibits a primary amino
acid sequence and major motifs and domains characteristic of the FNR family of
proteins, including an effector binding domain with at least three of the four
cysteines known to coordinate an [4Fe-4S]2+ center, a dimerization domain, and a
DNA binding domain. Western blotting with antibodies against Escherichia coli
FNR (FNREC) recognized FNRAF. FNRAF was able to drive expression from the
FNR-responsive E. coli promoter PnarG, suggesting that it is functionally active
as an FNR-like protein. Upon air exposure, FNRAF demonstrated an unusual lack of
sensitivity to oxygen compared to the archetypal FNREC. Comparison of the
primary amino acid sequence of FNRAF with that of other natural and mutated
FNRs, including FNREC, coupled with an analysis of the predicted tertiary
structure of FNRAF using the crystal structure of the related FNR from
Aliivibrio fisheri as a template revealed a number of amino acid changes that
could potentially stabilize FNRAF in the presence of oxygen. These include a
truncated N terminus and amino acid changes both around the putative Fe-S
cluster coordinating cysteines and also in the dimer interface. Increased O2
stability could allow At. ferrooxidans to survive in environments with
fluctuating O2 concentrations, providing an evolutionary advantage in natural,
and engineered environments where oxygen gradients shape the bacterial
community.
