Specific and reversible inhibition of the blunt-end joining activity of the T4 DNA ligase

Specific, complete and reversible inhibition of the
joining of blunt ended DNA duplexes catalyzed by the T4 DNA ligase can be obtained by using ATP, the enzyme cofactor, at concentrations of 5 mM and higher.
On cohesive DNA ends, 5 mM ATP, which completely inhibits blunt end ligation, brings about only a limited (8%) reduction in the level of joining obtainable under optimal ATP concentra tion (0,5 mM or lower). A similar but less drastic uncoupling of the two kinds of joining can b$+achieved at lower ATP con_ centration(2,5mM)using1mMMg .
The joining of DNA blunt ends can also be inhibited almost completely by 10 mM spermidine, without noticeable effect on the joining of cohesive termini.