19 julio 2019

Identification and Unusual Properties of the Master Regulator FNR in the Extreme Acidophile Acidithiobacillus ferrooxidans.

DOI : 10.3389/fmicb.2019.01642

David S. Holmes

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


Investigadores Participantes del Centro

Extremophile Genomics and Astrobiology Laboratory

Doctor en Bioquímica, California Institute of Technology, USA.



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