Practice question

Escherichia coli DNA ligase is a 74 kilodalton NAD+-dependent enzyme that seals single-strand nicks in double-stranded DNA by joining 3'-hydroxyl and 5'-phosphate groups. It functions optimally between 25 and 37 degrees Celsius and is irreversibly denatured upon exposure to 70 degrees Celsius. Its physiological role includes joining Okazaki fragments and repairing DNA damage. Critically, the enzyme exhibits poor activity on blunt-ended DNA substrates and strongly prefers cohesive ends with complementary overhangs. Therefore, attributing efficient blunt-end ligation capability to Escherichia coli ligase misrepresents its substrate specificity compared with T4 DNA ligase.