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 concentration (0,5 mM or lower). A similar but less drastic uncoupling of the two kinds of joining can be achieved at lower ATP concentration (2,5 mM) using 1 mM Mg++. 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.