Consistent with the observation that AXP stimulates the cos - cleavage reaction ( Figure 2B ) , the data presented in Figure 3A demonstrate that both ATP and ADP stimulate DNA packaging when included in step 1 of the reaction , i.e. , during maturation complex assembly . Of note , selective packaging of the matured D L end is observed under these conditions . In contrast , while IHF also stimulates the cos - cleavage reaction ( Figure 2D ) , the host protein alone does not support the transition to packaging ( Figure 3B ) . When AXP and IHF are both included in step 1 , additivity is observed and the host protein stimulates AXP - promoted packaging 2 - 3 - fold ( compare Figure 3A , B ) . Importantly , both must be present during maturation complex assembly ( step 1 ) as addition of IHF or ADP in step 2 has no effect on packaging ( data not shown ) . Similar to the cos - cleavage reaction , AMP - PCP ( 1 mM ) partially stimulates the reaction while AMP has little effect ( Figure 3C ) .