A pharmaceutical researcher develops a novel antibacterial drug that works by inhibiting exonuclease activity during DNA replication. When actively dividing Escherichia coli is exposed to the drug, enzyme-mediated nucleotide removal in the 5' to 3' direction is impaired, leading to inhibition of bacterial growth. Which of the following enzymes is the most likely target of this drug?
Show Explanatory Sources
DNA polymerases are the primary enzymes responsible for DNA synthesis, which occurs in the 5' to 3' direction. Prokaryotes such as Escherichia coli have 3 major DNA polymerases: I, II, and III. DNA replication requires a high degree of fidelity to preserve the genetic code in daughter cells and prevent potentially lethal mutations. The first line of defense against DNA replication errors is accomplished by the 3' to 5' proofreading exonuclease activity of all 3 DNA polymerases; upon detecting a replication error, they can reverse direction by one base pair and excise the mismatched base before continuing with DNA polymerization.
DNA polymerase I is unique as it is the only prokaryotic polymerase that also has 5' to 3' exonuclease activity. This activity functions to remove the RNA primer created by primase and repair damaged DNA sequences.
(Choice B) DNA polymerase III has 3' to 5' exonuclease activity; however, it does not possess 5' to 3' exonuclease activity.
(Choice C) The enzyme topoisomerase II, also known as DNA gyrase in prokaryotes, relieves tension created during DNA strand unwinding by introducing negative supercoils into the circular DNA. Fluoroquinolones (eg, ciprofloxacin) are a class of antibiotics that work by inhibiting DNA gyrase.
(Choice D) Before DNA replication begins, helicase unwinds the DNA double helix, facilitating separation of the 2 DNA strands which are then stabilized by single-stranded DNA-binding proteins.
(Choice E) Okazaki fragments of the lagging strand are bound together by the enzyme ligase.
(Choice F) DNA polymerases cannot begin synthesizing complementary DNA on a single-stranded template without an RNA primer. Primase is an RNA polymerase that synthesizes this primer, which is made up of short stretches of RNA base paired to the DNA template.
Educational objective:
In prokaryotes, only DNA polymerase I has both 5' to 3' and 3' to 5' exonuclease activities. The 3' to 5' exonuclease activity allows for a proofreading function during DNA replication, while the 5' to 3' exonuclease activity is involved in removal of RNA primers and repair of damaged DNA sequences.