Chloroplast-encoded small subunit extensions reshape the Chlamydomonas chlororibosome

Area of Science:

• Molecular Biology
• Structural Biology
• Algal Biology

Background:

• Chloroplast ribosomes (chlororibosomes) are essential for photosynthesis but their structure is poorly understood in non-flowering plants.
• Understanding chlororibosome structure is key to deciphering chloroplast gene expression and its evolution.

Purpose of the Study:

• To determine the in situ and high-resolution structure of chlororibosomes from the green alga Chlamydomonas reinhardtii.
• To investigate the structural basis for chlororibosome function and diversity.

Main Methods:

• Combined in situ cryo-electron tomography and single-particle cryo-electron microscopy.
• Subtomogram averaging of chlororibosomes in their native thylakoid membrane environment.
• High-resolution structural determination of isolated chlororibosomes bound to translation factors or tRNA.

Main Results:

• Revealed chlororibosomes free in the stroma and membrane-tethered, in distinct translational states.
• Discovered a novel 'arm' domain on the small subunit, formed by chloroplast-encoded elements.
• The arm domain localizes to mRNA channels, suggesting roles in mRNA stabilization and polysome organization.

Conclusions:

• Algal chlororibosomes exhibit unexpected structural diversity compared to those in flowering plants.
• The novel arm domain highlights significant diversification in chloroplast translation mechanisms.
• These findings advance our understanding of chloroplast protein synthesis and its evolutionary trajectory.