Polysomes of eukaryotic cells observed by electron microscopy

Area of Science:

• Cell Biology
• Molecular Biology
• Genetics

Background:

• Polysomes, the complexes of messenger RNA (mRNA) and ribosomes involved in protein synthesis, are typically depicted as linear structures.
• The precise in vivo architecture and conformational dynamics of polysomes within eukaryotic cells remain incompletely understood.
• Previous studies have primarily focused on the linear arrangement of ribosomes translating mRNA.

Purpose of the Study:

• To investigate the three-dimensional shapes of polysomes in various eukaryotic cell types.
• To determine if polysomes exhibit non-linear, potentially circular, conformations within the cellular environment.
• To ascertain whether observed polysome shapes are intrinsic cellular features or artifacts of experimental preparation.

Main Methods:

• Surface spreading technique applied to eukaryotic cells for polysome visualization.
• Examination of polysomes in unicellular protozoan Tetrahymena cells.
• Analysis of polysomes in mammalian cell lines including rabbit reticulocytes, MDCK, BHK, and HeLa cells.
• Isolation and partial purification of polysomes from Tetrahymena for comparative analysis.

Main Results:

• Polysomes were observed in diverse shapes, including ring, 8-shaped, and caterpillar-like forms.
• These varied shapes suggest that eukaryotic cells contain fundamentally circular polysomes.
• Isolated and purified polysomes from Tetrahymena exhibited similar structural features to those observed in spread cells, supporting their biological relevance.

Conclusions:

• Eukaryotic cells harbor polysomes with fundamentally circular structures.
• The observed circular and varied polysome shapes are intrinsic cellular features, not artifacts of the surface spreading technique.
• This finding challenges the traditional linear model and suggests a more complex architecture for active protein synthesis machinery in eukaryotes.