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.
-Which observation would NOT support the hypothesis of gene transfer from eukaryotes to prokaryotes described in the passage?

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.
-Which observation would NOT support the hypothesis of gene transfer from eukaryotes to prokaryotes described in the passage?

Quizplus · Accepted Answer

11ecba2d_0f84_5c50_a3bf_a33b07a00778_MD0008_00 According to the passage, scientists proposed that a prolonged symbiosis period could induce gene transfer from eukaryotes to prokaryotes. In the given example, P. leiognathi (symbiont) was believed to have acquired the gene coding for Cu/Zn SOD from its eukaryotic host (ponyfish) after an extended period of symbiosis. The passage states that Cu/Zn SOD is typically found in the cytosol of eukaryotes; therefore, scientists concluded that SODs found in P. leiognathi were most likely acquired from ponyfish. However, the discovery of Cu/Zn SODs in other free-living bacterial species with no known eukaryotic symbiotic hosts would argue against the hypothesis of eukaryotic to prokaryotic gene transfer due to symbiosis. (Choices A and B) The amino acid residues in the catalytic domain (active site) of an enzyme are involved in protein function because they catalyze the enzymatic reaction. In contrast, residues in noncatalytic domains constitute a large percentage of the gene (and protein) sequence but do not contribute primarily to catalytic function. Because sequence similarity indicates common evolutionary ancestry, the hypothesis of gene transfer from eukaryotes to prokaryotes would be supported by the discovery of a similar SOD gene sequence between ponyfish and P. leiognathi or a similar amino acid sequence in the noncatalytic domains of ponyfish and P. leiognathi SODs. (Choice D) The discovery of Cu/Zn SODs in other free-living bacterial species with known eukaryotic symbiotic hosts reinforces the hypothesis of eukaryotic to prokaryotic gene transfer. Educational objective: The endosymbiotic theory explains how primitive eukaryotic anaerobes engulfed ancient aerobic prokaryotes, and consequently acquired the ability to produce energy through oxidative phosphorylation.

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