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

Passage
Eukaryotic organelles such as mitochondria and chloroplasts are believed to be relics of formerly free-living prokaryotes.  The transition from a hypoxic (low O₂) to an oxic atmosphere (21% O₂) 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 (O₂⁻) radicals, a form of ROS, are sequestered by SODs and converted into less toxic hydrogen peroxide (H₂O₂) and O₂ 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.
-SOD mRNA in P. leiognathi encodes a signal sequence that directs the transport of the SOD protein for secretion.  This signal sequence will direct SOD proteins to which structure in P. leiognathi?

A) Smooth endoplasmic reticulum
B) Plasma membrane
C) Mitochondrial outer membrane
D) Golgi body

Correct Answer:

Verified

Unlock this answer now
Get Access to more Verified Answers free of charge

Access For Free