Chloroplast-encoded small subunit extensions reshape the Chlamydomonas chlororibosome

Area of Science:

• Chloroplast biology
• Molecular and structural biology
• Algal research

Background:

• Chloroplast ribosomes (chlororibosomes) are essential for synthesizing proteins vital to photosynthesis.
• The structural diversity of chlororibosomes, particularly in non-flowering plants, is not well understood.

Purpose of the Study:

• To determine the structure of the chlororibosome from the unicellular green alga *Chlamydomonas reinhardtii*.
• To investigate the structural variations and functional implications of chlororibosomes in their native environment.

Main Methods:

• In situ cryo-electron tomography (cryo-ET) to visualize chlororibosomes within the algal cell.
• Single-particle cryo-electron microscopy (cryo-EM) for high-resolution structural determination.
• Subtomogram averaging to analyze chlororibosomes in distinct translational states.

Main Results:

• A novel 'arm' domain was identified on the small subunit of *Chlamydomonas reinhardtii* chlororibosomes.
• This arm domain, composed of chloroplast-encoded insertions and specific ribosomal proteins, is located near mRNA channels.
• Chlororibosomes were observed free in the stroma and associated with thylakoid membranes in various translational states.

Conclusions:

• Algal chlororibosomes exhibit unexpected structural variations compared to those in flowering plants.
• The newly discovered arm domain likely plays a role in mRNA stabilization and chloroplast polysome organization.
• Chloroplast translation has undergone substantial diversification across different photosynthetic lineages.