For a proposed mechanism for DNA ligation involving the E coli enzyme with NAD+ as cosubstrate see: Horton, H.R., Moran, L.S., Ochs, R.S., Rawn.J.D. and Scrimgeour, K.G.; Principles of Biochemistry, 3rd Ed., p.643, Prentice Hall, Saddle River, N.J., 2002.