Passage
Eukaryotic organelles such as mitochondria and chloroplasts are believed to be relics of formerly free-living prokaryotes. The transition from a hypoxic (low O 2 ) to an oxic atmosphere (21% O 2 ) is said to have enabled primitive eukaryotic anaerobes to engulf ancient aerobic prokaryotes and consequently acquire the ability to produce energy through oxidative phosphorylation. This endosymbiotic theory of eukaryotic evolution also postulates that endosymbiosis resulted in larger eukaryotic genomes, which originated from the partial transfer of mitochondrial genes to the nuclear genome. On integration into the host genome, mitochondria-derived genes became indistinguishable from the original nuclear genes.Researchers have alternatively proposed that after a prolonged period of symbiosis, there is a possibility of gene transfer from eukaryotes to prokaryotes. This hypothesis was initially supported when copper/zinc (Cu/Zn) superoxide dismutase (SOD), a metalloprotein confined to the cytosol of eukaryotic cells, was found in Photobacterium leiognathi. The free-living bacterium P. leiognathi is also a known symbiont of ponyfish, a small fish species native to the Indian and Pacific Oceans.SODs are antioxidant enzymes that serve as the cell's first line of defense against reactive oxygen species (ROS). ROS produced by the electron transport chain damage proteins by oxidizing amino acid residues and metal ions on prosthetic groups, but can accumulate during times of biochemical and environmental stress. Superoxide (O 2 − ) radicals, a form of ROS, are sequestered by SODs and converted into less toxic hydrogen peroxide (H 2 O 2 ) and O 2 gas.
Adapted from Bannister JV, Parker MW. The presence of a copper/zinc superoxide dismutase in the bacterium Photobacterium leiognathi: a likely case of gene transfer from eukaryotes to prokaryotes. Proc Natl Acad Sci USA. 1985;82(1):149-52.
-Investigators used a ribosome profiling technique to measure the level of SOD protein synthesis in P. leiognathi and ponyfish cells during ROS accumulation. P. leiognathi cells were found to contain greater numbers of ribosome/SOD mRNA complexes than ponyfish cells. What is the best explanation for this result?

Question

Passage
Eukaryotic organelles such as mitochondria and chloroplasts are believed to be relics of formerly free-living prokaryotes.  The transition from a hypoxic (low O 2 ) to an oxic atmosphere (21% O 2 ) is said to have enabled primitive eukaryotic anaerobes to engulf ancient aerobic prokaryotes and consequently acquire the ability to produce energy through oxidative phosphorylation.  This endosymbiotic theory of eukaryotic evolution also postulates that endosymbiosis resulted in larger eukaryotic genomes, which originated from the partial transfer of mitochondrial genes to the nuclear genome.  On integration into the host genome, mitochondria-derived genes became indistinguishable from the original nuclear genes.Researchers have alternatively proposed that after a prolonged period of symbiosis, there is a possibility of gene transfer from eukaryotes to prokaryotes.  This hypothesis was initially supported when copper/zinc (Cu/Zn) superoxide dismutase (SOD), a metalloprotein confined to the cytosol of eukaryotic cells, was found in Photobacterium leiognathi.  The free-living bacterium P. leiognathi is also a known symbiont of ponyfish, a small fish species native to the Indian and Pacific Oceans.SODs are antioxidant enzymes that serve as the cell's first line of defense against reactive oxygen species (ROS).  ROS produced by the electron transport chain damage proteins by oxidizing amino acid residues and metal ions on prosthetic groups, but can accumulate during times of biochemical and environmental stress.  Superoxide (O 2 − ) radicals, a form of ROS, are sequestered by SODs and converted into less toxic hydrogen peroxide (H 2 O 2 ) and O 2 gas.
Adapted from Bannister JV, Parker MW. The presence of a copper/zinc superoxide dismutase in the bacterium Photobacterium leiognathi: a likely case of gene transfer from eukaryotes to prokaryotes. Proc Natl Acad Sci USA. 1985;82(1):149-52.
-Investigators used a ribosome profiling technique to measure the level of SOD protein synthesis in P. leiognathi and ponyfish cells during ROS accumulation.  P. leiognathi cells were found to contain greater numbers of ribosome/SOD mRNA complexes than ponyfish cells.  What is the best explanation for this result?

Quizplus · Accepted Answer

11ecba2d_0f87_4282_a3bf_91e798476e50_MD0008_00 Prokaryotic cells have no nucleus or nuclear envelope, and due to the lack of this physical barrier between DNA and the ribosomal machinery, transcription and translation occur simultaneously in the cytoplasm. Ribosome profiling was used to measure the level of SOD translation by detecting the number of SOD mRNA/ribosome complexes found within ponyfish (a eukaryote) and P. leiognathi(a prokaryote). The question states that P. leiognathi cells have a greater number of ribosome/SOD mRNA complexes compared to ponyfish cells. This rapid protein synthesis is likely due to the fact that all SOD mRNA transcripts in P. leiognathi quickly interact with ribosomes before being fully transcribed (ie, transcription and translation are coupled). (Choice A) Both eukaryotic and prokaryotic mRNAs can be bound by multiple ribosomes in the cytoplasm. (Choice B) Ponyfish mRNA is translated by the 80S ribosome whereas P. leiognathi mRNA is translated by the 70S ribosome. In this scenario, no information is given to suggest that ribosome affinity is a factor for SOD synthesis. (Choice C) In eukaryotic cells (eg, ponyfish), transcription of mRNA and its modification (eg, addition of the 5′ cap, 3′ poly-A tail, splicing) typically occur in the nucleus. The modifications, which result in a mature mRNA molecule, increase the stability of the transcript and prevent its degradation in the cytoplasm. Because the nucleus is separated from the cytoplasm by a nuclear envelope, mRNA must first be transported through nuclear pores to the cytoplasm, where it is translated by ribosomes. Educational objective: Prokaryotic cells have no nucleus; therefore, transcription and translation occur simultaneously in the cytoplasm (ie, translation begins before the mRNA is fully transcribed). By contrast, in eukaryotic cells transcription and post-transcriptional modifications occur in the nucleus, but translation occurs in the cytoplasm.

Passage Eukaryotic Organelles Such as Mitochondria and Chloroplasts Are Believed to to Be