Adaptation of Fe-S Cluster Assembly to Rising O₂ Levels over Geological Time

Area of Science:

• Biogeochemistry
• Molecular Evolution
• Biochemistry

Background:

• The Great Oxidation Event (GOE) significantly impacted early life, leading to the extinction of many anaerobic microorganisms.
• Iron-sulfur (Fe-S) clusters are vital cofactors for all life forms, but their adaptation to increasing oxygen levels remains poorly understood.
• The Sulfur Utilization factor (SUF) pathway is a primary route for Fe-S cluster biogenesis.

Purpose of the Study:

• To investigate the molecular adaptation of the SUF pathway, specifically the SufE protein, to rising oxygen levels during Earth's history.
• To elucidate the evolutionary trajectory of SufE and its partner SufS in response to the Great Oxidation Event.
• To understand the biochemical and structural basis for SufE's resilience under oxidative stress.

Main Methods:

• Molecular clock dating to establish the evolutionary timeline of SufE and SufS.
• Reconstruction and biochemical assays of ancestral SufS/SufE complexes.
• Bacterial growth experiments using *Escherichia coli* mutants.
• Enzyme structure prediction analysis.

Main Results:

• SufE originated around 2.67 billion years ago (Ga) and diversified near the GOE (~2.14 Ga).
• Ancestral SufS/SufE complexes showed increased oxygen tolerance, with the GOE versions active at higher O₂ concentrations than their last common ancestor (LCA) counterparts.
• Overproduction of GOE SufE or SufS/SufE enhanced growth in oxygen-sensitive *E. coli* mutants more effectively than LCA versions.
• Structural analysis indicated amino acid substitutions in key catalytic sites enabled adaptation.

Conclusions:

• The SufE protein evolved molecular mechanisms to maintain function under oxidative stress, facilitating Fe-S cluster assembly during the Great Oxidation Event.
• These adaptations provided crucial resilience for life adapting to fluctuating redox conditions throughout Earth's history.
• The study reveals the molecular basis for the coevolution of the biosphere and geosphere in response to rising oxygen levels.