Ellis Engelsberg and his colleagues examined the regulation of genes taking part in the metabolism of arabinose, a sugar (E. Engelsberg et al. 1965. Journal of Bacteriology 90:946-957). Four structural genes encode enzymes that help metabolize arabinose (genes A, B, D, and E). An additional sequence C is linked to genes A, B, and D. These are in the order D- A- B- C. Gene E is distant from the other genes. Engelsberg and his colleagues isolated mutations at the C sequence that affected the expression of structural genes A, B, D, and E. In one set of experiments, they created various genotypes at A and C and determined whether arabinose isomerase (the enzyme encoded by gene A) was produced in the presence or absence of arabinose (the substrate of arabinose isomerase) by cells with these genotypes. Results from this experiment are shown in the following table, where a plus sign (+) indicates that the arabinose isomerase was synthesized and a minus sign (-) indicates that the enzyme was not synthesized. Arabinose Arabir Genotype absent prese 1. C* A* 2. C A* 3. C A*/C* 4. CA/C A*

A mitochondrion is a cellular organelle that serves as the site for cellular respiration and energy production (ATP or Adenosine Triphosphate). It is a double-membrane-bound organelle and is only present in eukaryotic cells.