Nucleo Chloroplastic Interaction
- Nuclear Genes Cooperate with Chloroplast Genes in Making Chloroplast proteins chloroplasts carry less than 10 percent of the genetic information required to assemble chloroplast.
- Most of the polypeptides needed by chloroplasts are encoded by nuclear genes, synthesized on free cytoplasmic ribosomes, released into the cytosol, and imported into chloroplasts using the targeting mechanisms.
- Polypeptides destined for uptake by chloroplasts contain signal sequences that bind to receptor proteins located in the outer membrane of chloroplast.
- After binding, the polypeptide chain is inserted into or transferred across the chloroplast membranes, depending on its ultimate location in the organelle.
- The signal sequences are then cleaved by proteases to generate mature protein molecules.
- Since some genes coding for chloroplast polypeptides occur in the nucleus while others reside within the chloroplast the question arises as to how the expression of the genes located in the organelles is coordinated with those residing in the nucleus.
- This issue is especially important for proteins that require polypeptides encoded by genes located in two different compartments,
- For example, rubisco requires one polypeptide encoded by chloroplast DNA and one polypeptide encoded by nuclear DNA.
- The mechanism that coordinates the synthesis of polypeptides made in separate locations but destined for assembly into the same protein is not well understood.
- In chloroplasts, some insights have emerged from studies involving the synthesis of the large and small subunits of rubisco.
- If an inhibitor of cytoplasmic protein synthesis is added to intact cells to block formation of the small rubisco subunit by cytoplasmic ribosomes, synthesis of the large rubisco subunit by chloroplasts eventually slows down as well, even though the inhibitor has no direct effect on protein synthesized within the organelles are manufactured in the cytoplasm and then imported? The answer to this question may lie in the evolutionary history of chloroplasts.
- Chloroplast is thought to have evolved from bacteria that were ingested by eukaryotic cells a billion or more years ago.
- As time progressed and the ingested bacteria evolved into chloroplasts, most of the bacterial genes were either lost or transferred to the nucleus.
- But why weren’t all the chloroplast genes relocated to the nucleus?
- The answer may be related to the fact that many polypeptides encoded by chloroplast DNA are hydrophobic molecules that reside in the thylakoid membrane.
- Because it is thermodynamically unfavorable to place a hydrophobic molecule in an aqueous environment, it would be disadvantageous to synthesize hydrophobic polypeptides on free cytoplasmic ribosomes and then release them into the cytosol prior to transport into the chloroplast.
- Such polypeptides are therefore synthesized on ribosomes attached to the thylakoid membrane; this arrangement allows the polypeptides to be inserted directly into the membrane as they are being synthesized, just as ribosomes bound to the ER insert their newly synthesized polypeptides directly into the ER membrane.
- In most cases the gene that codes for a given chloroplast polypeptide occurs either in the organelle or in the nucleus, but not both.
- However, exceptions do occur. Most of these nuclear sequences contain fragments of different chloroplastic genes that are fused together, making them genetically inactive. Thus when similar sequences are present in both nuclear and chloroplast DNA, in some instances the nuclear sequences are functional and in other cases the chloroplast sequences are functional.
