Ribonucleotide reduction - horizontal transfer of a required function spans all three domains

Area of Science:

• Evolutionary biology
• Genomics
• Biochemistry

Background:

• Ribonucleotide reductases (RNRs) catalyze the essential de novo synthesis of deoxyribonucleotides for DNA.
• Three classes of RNRs exist, with varied distribution across Bacteria, Archaea, and Eukaryotes.
• The evolutionary origins and distribution patterns of RNR classes are not fully understood.

Purpose of the Study:

• To investigate the evolutionary history of RNRs across the three domains of life.
• To determine if RNR classes were present in the Last Universal Common Ancestor (LUCA) or acquired via horizontal gene transfer (HGT).
• To assess the impact of environmental factors on RNR class distribution.

Main Methods:

• Phylogenetic analyses of RNR genes.
• Comparative genomics to trace gene origins and transfers.
• Examination of environmental factors influencing RNR distribution.

Main Results:

• The Last Eukaryotic Common Ancestor (LECA) possessed a bacterial-derived class I RNR.
• Archaeal class I RNRs originated from independent HGT events from bacteria.
• Eukaryotic class I RNR has been transferred back to bacteria (Bacteroidetes), demonstrating eukaryote-to-bacteria HGT.
• Eukaryotic class II and III RNRs also appear to be of bacterial origin, with subsequent eukaryotic-to-eukaryotic transfers.
• The RNR in the last common ancestor of Archaea and eukaryotes may have been a dimeric class II RNR, though HGT complicates definitive conclusions.

Conclusions:

• Horizontal gene transfer (HGT) has profoundly influenced the RNR repertoire across all life domains.
• Class I RNRs likely evolved in bacteria and spread to archaea and eukaryotes via HGT.
• The ancient presence of RNR classes II and III in LUCA remains uncertain due to extensive HGT.
• Environmental factors, particularly oxygen presence, likely drive the ongoing selection and distribution of RNR classes.