Supersized Ribosomal RNA Expansion Segments in Asgard Archaea

Area of Science:

• Molecular Biology
• Evolutionary Biology
• Biochemistry

Background:

• The ribosome's core structure, composed of ribosomal RNA (rRNA) and proteins, is fundamental to all life and reveals evolutionary relationships.
• Eukaryotic ribosomes feature large expansion segments (ESs) on their rRNA, increasing size, but the origin of these ESs in Archaea remains unclear.
• Asgard archaea, including Lokiarchaeota and Heimdallarchaeota, are considered the closest archaeal relatives to Eukarya.

Purpose of the Study:

• To investigate the structure and evolutionary origins of large ribosomal subunit (LSU) rRNA expansion segments in Asgard archaea.
• To understand how these expansion segments contribute to the size difference between prokaryotic and eukaryotic ribosomes.
• To test the accretion model of ribosomal evolution by examining the topology of expansion segments.

Main Methods:

• Chemical footprinting experiments to determine the structure of Lokiarchaeota expansion segment 39 (ES39).
• Covariation and sequence analysis to study the evolution of Asgard archaeal ES39 and ES9.
• Comparative analysis of Asgard and eukaryotic ES39 structures to identify conserved features.

Main Results:

• Asgard archaea (Lokiarchaeota and Heimdallarchaeota) possess supersized expansion segments (ES9 and ES39) in their LSU rRNA, bridging the size gap between prokaryotic and eukaryotic ribosomes.
• Asgard ES39s exhibit distinct sequences and structures compared to eukaryotic ES39s, featuring more and longer helices.
• A conserved three-way junction in Asgard ES39s matches the topology of eukaryotic ES39s, supporting the accretion model.

Conclusions:

• The discovery of large expansion segments in Asgard archaea provides a crucial link in understanding ribosome evolution.
• The findings suggest that eukaryotic expansion segments evolved through the accretion of pre-existing structural elements.
• This research sheds light on the early evolution of the eukaryotic ribosome and its relationship to archaeal ancestors.