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Related Concept Videos

Export of Mitochondrial and Chloroplast Genes02:19

Export of Mitochondrial and Chloroplast Genes

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A eukaryotic cell can have up to three different types of genetic systems: nuclear, mitochondrial, and chloroplast. During evolution, organelles have exported many genes to the nucleus; this transfer is still ongoing in some plant species. Approximately 18% of the Arabidopsis thaliana nuclear genome is thought to be derived from the chloroplast’s cyanobacterial ancestor, and around 75% of the yeast genome derived from the mitochondria’s bacterial ancestor. This export has occurred...
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Protein Transport to the Inner Chloroplast Membrane01:18

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Proteins targeted to the inner chloroplast membrane, or plastid proteins, are transported by two general pathways: the stop-transfer and the re-insertion or post-import pathways. Most plastid proteins carry N-terminal transit sequences and internal import sequences targeting it to the specific chloroplast subcompartment. Proteins targeted by the stop-transfer pathway have internal hydrophobic sequences that inhibit their translocation into the stroma. As a result, these precursors are arrested...
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Protein Transport to the Outer Chloroplast Membrane01:11

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Chloroplast outer membrane proteins encoded by the nucleus are synthesized in the cytosol. Soon after synthesis, they bind cytosolic factors such as 14-3-3 protein and the Hsp70 chaperones that keep these precursors in an unfolded state until their translocation.
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Protein Transport to the Stroma01:24

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Chloroplasts are triple membrane structures with an outer membrane, an inner membrane, and a thylakoid membrane, each containing distinct metabolite transporters, membrane translocons, and enzymes. Appropriate sorting and translocating these proteins to their correct membrane systems is essential for chloroplast function.
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The present-day mitochondrial and chloroplast genomes have retained some of the characteristics of their ancestral prokaryotes and also have acquired new attributes during their evolution within eukaryotic cells. Like prokaryotic genomes, mitochondrial and chloroplast genomes neither bind with histone-like proteins nor show complex packaging into chromosome-like structures, as observed in eukaryotes. Unlike mitotic cell divisions observed in eukaryotic cells, mitochondria and chloroplasts...
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Thylakoids are membrane-bound sac-like structures within the chloroplast that serve as sites for photosynthesis. Thylakoid lumen contains many electron transport proteins and is enclosed by a thylakoid membrane rich in the light-harvesting complex. Proteins targeted to the thylakoids are transported as precursors and are sorted by the general TOC/TIC import pathway. Once the precursor reaches the stroma, stromal processing peptidases remove their transit signal and expose thylakoid signal...
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Does mRNA targeting explain gene retention in chloroplasts?

Wolfgang R Hess1, Annegret Wilde2, Conrad W Mullineaux3

  • 1Genetics and Experimental Bioinformatics, Faculty of Biology, University of Freiburg, Freiburg, Germany.

Trends in Plant Science
|October 23, 2024
PubMed
Summary

Chloroplasts retain some genes from cyanobacteria for photosynthesis. We propose the "mRNA targeting hypothesis," suggesting retained genes are essential for mRNA to target thylakoid membranes for protein assembly.

Keywords:
chloroplastcyanobacteriaendosymbiosisevolutionphotosynthesisthylakoid membrane

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Area of Science:

  • Plant Biology
  • Molecular Biology
  • Evolutionary Biology

Background:

  • Plastids, originating from cyanobacteria, have transferred most genes to the nucleus but retain essential organellar genes.
  • Previous hypotheses for gene retention include codon bias, protein import issues, and redox regulation needs.
  • The function of retained genes within the plastid, particularly related to photosynthesis, remains incompletely understood.

Purpose of the Study:

  • To propose a novel hypothesis explaining the retention of core photosynthetic genes within chloroplasts.
  • To investigate the role of mRNA localization and function in gene retention during plastid evolution.
  • To connect gene retention to the assembly of photosynthetic protein complexes at the thylakoid membrane.

Main Methods:

  • This is an opinion article, presenting a hypothesis based on existing literature.
  • It reviews studies on cyanobacteria and chloroplast gene expression.
  • It focuses on the proposed 'mRNA targeting hypothesis' and its implications.

Main Results:

  • The 'mRNA targeting hypothesis' is proposed, suggesting mRNA features are key to gene retention.
  • Evidence from cyanobacteria indicates mRNAs for photosynthetic proteins possess membrane-targeting capabilities.
  • These mRNA features are crucial for coordinating protein complex assembly at the thylakoid surface.

Conclusions:

  • The retention of core photosynthetic genes in chloroplasts is explained by the necessity of direct mRNA involvement at the thylakoid membrane.
  • mRNA targeting and its role in early assembly steps provide a compelling explanation for organellar gene retention.
  • This hypothesis integrates gene expression, protein targeting, and evolutionary retention in chloroplasts.