Do Chloroplasts Have Ribosomes? 9 Facts You Should Know


Chloroplast ribosome is also referred to as “plasto-ribosomes” or “chloro-ribosome”. This article will provide you 9 facts about chloroplast ribosomes.

Chloroplasts are double-membraned cellular organelle of plants that helps in energy conversion and light-dependent reactions. These organelles contain their own protein synthesis machinery i.e., ribosomes.

Do all chloroplasts have ribosomes ?

Chloroplasts are the powerhouse for plants. It can absorb light energy and perform carbohydrate metabolism. They are semiautonomous since they have their own genome (extra-nuclear DNA – chloroplast DNA) and ribosomes. These ribosomes play key roles in translation encoded by chloroplast DNA.

Yes, all chloroplast has ribosomes. Chloroplasts are double-membraned cellular organelle of plants that helps in energy conversion and light-dependent reactions. These organelles contain their own protein synthesis machinery i.e., ribosomes. The chloroplast ribosome is a characteristic organellar ribosome that synthesizes protein encoded by the chloroplast genome.

do chloroplast have ribosomes
Chloroplast and its ribosomal subunits

Chloroplast ribosomes are structurally similar to prokaryotic 70S ribosomes and greatly distinct from 80S eukaryotic ribosomes. In higher plants and algae, plastid ribosomes account about 25% of the total ribosomes. These ribosomes synthesize about 80-100 polypeptides, specially synthesizes the larger subunit of the enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco). Constratingly, those of red algae (Rhodophyta) chloroplast ribosomes also contribute to cellular protein synthesis, translating >200 polypeptides.

The chloroplast genome has 120-130 genes, primarily involved in photosynthesis.

Chloroplast DNASize
Codium fragile85 kb
Chlamydomonas moewusii292 kb
Acetabularia2000 kb
Nicotiana844
Pelargonium2188 kb

Why do chloroplasts have ribosomes ?

Chloroplast ribosomes play vital roles in translating protein encoded by chloroplast DNA.

The ribosomes of chloroplast originated from an ancient photosynthetic bacterium that was engulfed by a eukaryotic host. The remnants of that ancient bacteria were retained in the present-day chloroplast. It synthesizes various important proteins for chloroplast necessary for photosynthesis. It translates larger subunit of Rubisco and also contributes to cellular protein biosynthesis in some cases.

do chloroplast have ribosomes
Image credit Wikimedia
Chloroplast endosymbiosis

It was believed that one anaerobic bacterium once engulfed an aerobic bacterium but not digested completely. But established a symbiotic relationship. The aerobic bacterium eventually converted into mitochondria. In addition to it, this heterotrophic bacterium engulfed a photosynthetic cyanobacterium which can convert light energy into ATP. This photosynthetic cell became the chloroplast thus retained most of the characteristic of photosynthetic bacterium.

Chloroplast ribosome function

These are the most distinctive organelles of plant cells involved in various metabolic pathways. The functions are as follows

  1. Synthesizes larger subunit of the enzyme Rubisco
  2. Helps in differentiation of plants and algae
  3. Contributes to cellular protein synthesis
  4. Synthesizes other ribosomal proteins

Chloroplast ribosome characteristics

  1. Chloroplasts contain 70S ribosomes.
  2. The plastid ribosome bears a significant similarity with cyanobacterial ribosomes.
  3. The chloro-ribosomes contain 58 ribosomal proteins.
  4. The ribosomal RNA of chloroplast contains 4,524 nucleotides (reported in Spinach) which are of the same length as the bacterial ribosome.
  5. Chloroplasts contain ribosomes of prokaryotic origin which constitute about 30% of total leaf ribosomes.

Chloroplasts contain bacterial-type 70S which is composed of 50S and 30S subunits. 30S subunit interacts with initiation factors associated with protein synthesis via the Shine-Dalgarno sequence. While the 50S, the larger subunit helps in the elongation process coupled with elongation factors.  

The larger subunit of plastid ribosome 50S contains three rRNA which are 23S rRNA, 5S rRNA, and 4.5S rRNA, whereas 30S contains only one rRNA i.e., 16S rRNA.

The 50S subunit contains 33 proteins and 25 proteins are present in the 30S subunit. The ribosomal proteins present in 30S subunit are PRPS proteinsplastid-specific ribosomal proteins and 50S subunit proteins are PRPL proteins.  

The 16S rRNA of bacterial counterpart E.coli contains 1,542 nucleotides whereas in spinach it is about 1,491 nucleotides. Additionally, the combined size of the 50S subunit of spinach is 3,033 nucleotides which are larger than E.coli (3,024 nucleotides).  

Chloroplast ribosomes actively synthesize chloroplast membranal proteins, and also some ribosomal proteins. It also regulates light-dependent reactions and protein synthesis. The chloroplast ribosome contains soluble proteins that are synthesized by cytoplasmic ribosomes. These are transferred consequentially to the outer membrane of the plastid.

Do chloroplasts have 80s ribosomes

80S ribosome is a common type of ribosome.

80S ribosomes are characteristic ribosomes of eukaryotes, mainly found in cytoplasm or embedded into endoplasmic reticulum. The subunits are 40S smaller subunit and 50S larger subunit. The weight is approximately 4 -4.5 MDa with sedimentation coefficient is 80. They have about 16,000 amino acids with 73 proteins in larger subunit while 33 proteins in smaller one.

Do chloroplasts have 70s ribosomes

Chloroplasts are semiautonomous since they have their own DNA and unique ribosomes.

Yes, chloroplasts have 70S ribosomes. Such ribosomes are unique to chloroplast. Chloroplast and mitochondria, both the organelles have acquired ribosomes and perform the translation. Chloroplasts have 70S ribosomes which are similar to prokaryotic ribosomes.

Do chloroplasts have small ribosomes

Yes chloroplasts have smaller ribosomes. Chloroplast ribosomes are 70S ribosomes similar to prokaryotic ones which are smaller than eukaryotic 80S ribosomes. 70S ribosomes are smaller and simpler than 80S ribosomes. Size of 70S ribosome is 210 Å.

Chloroplast ribosome structure

Chloroplast ribosomes have similarities with prokaryotic ribosomes i.e., 70S ribosomes. However, it is markedly different from eukaryotic 80S ribosomes.

It contains two subunits 50S and 30S and other RNA components (23S, 16S, 5S and 4.5S rRNA). The molecular weight of the 50S and 30S subunits is about 1.6MDa and 9 MDa respectively.

The larger subunit of the ribosome, the 50S contains 5S and 23S rRNA and 34 proteins. 5S rRNA has 120 nucleotides while 23S consists of 2900 nucleotides. The smaller subunit has one rRNA i.e., 16S rRNA and 21 proteins. The distinct ribosomal RNA molecules form the core of the ribosome.

The 16S rRNA of Zea mays has a striking similarity with the 16S rRNA of E.coli. The spinach chloroplast rRNA contains 4,524 nucleotides. The 16S rRNA promotes mRNA binding and stabilizes the codon-anticodon interactions. The third component of the 50S subunit, 4.5S rRNA shows 55% sequence similarity with the bacterial 23S. 

do chloroplast have ribosomes
Image credit Wikipedia
50S subunit of the ribosome
do chloroplast have ribosomes
Image credit Wikipedia
30S subunit of the ribosome

Chloroplast ribosomes contain some specific proteins such as cS22 (PSRP2), cS23 (PSRP3), bTHXc (PSRP4) cL37 (PSRP5), and cL38 (PSRP6). The proteins cL37 and cL38 are associated with the larger subunit. cL37 has lysine and arginine-rich long helical polypeptide chains. The 70S subunit of chloroplast ribosome has N and C terminal-specific ribosomal proteins which exhibit striking architectural features.

Contrastingly bTHXc showed bacterial homology which is 26 amino acids containing basic protein, an integral part of the 30S subunit. The foot of the 30S subunit is truncated due to the addition of new protein and further protein extensions. cS22 is a 17 amino acid-containing protein with chaperone activity.

Chloroplast ribosome profiling

With the advancement of high-throughput analysis, chloroplast ribosome profiling serves as a tool to obtain significant information in contrast to classical methods. This method has been used in maize and Arabidopsis to understand translational dynamics.

Ribosome profiling is a genome-wide approach that helps to determine the protein synthesis rate, and translational activity and quantify the output amount. It exploits the stability of translating ribosomes while protecting them from nuclease activity, resulting in fragments called ribosome footprints.

do chloroplast have ribosomes
Ribosome profiling process From Wikipedia

This process involves next-generation sequencing that elucidates the in-vivo positions and the relative abundance of translating ribosomes. In chloroplast, ribosome profiling involves next-generation sequencing along with microarray hybridization.

Recently this process has attracted the attention of researchers and applied in various translation studies of prokaryotes and eukaryotes.  

Conclusion

Chloroplast is an unique double membrane bound organelle, play pivotal role in photosynthesis. Similar to mitochondria, chloroplast also contain its own genome and ribosomes. Chloroplast ribosomes are 70S organellar ribosomes that bears striking similarity with prokaryotic ribosomes. These ribosomes functions in synthesizing photosynthetic specific proteins like Rubisco.

Paurabi Das

I am a doctoral student of CSIR- CIMAP, Lucknow. I am devoted to the field of plant metabolomics and environmental science. I have completed my graduation from the University of Calcutta with expertise in Molecular Plant Biology and Nanotechnology. I am an ardent reader and incessantly developing concepts in every niche of biological sciences. I have published research articles in peer-reviewed journals of Elsevier and Springer. Apart from academic interests, I am also passionate about creative things such as photography and learning new languages. Let’s connect over Linkedin https://www.linkedin.com/in/paurabi-das-cimap26/

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