Figure 6.

Mechanism of bacteriophage λ excision. The specified apparent first-order rate constants are the lower limits for reactions at 60 nM Int, 6 nM IHF, and 250 nM Xis. All reactions within brackets are completed within the mixing time of 20 s and, therefore, have lower rate limits of 1/20 s=3.0 min⁻¹. Gray arrows denote processes that were not observed over the 60 min recording time and, therefore, have rate constants <∼0.017 min⁻¹. Abortive complexes could arise from any of the species within brackets. Covalent phosphotyrosine linkages (stars) resulting from DNA cleavage are likely to occur in attL before synapsis (not shown) and in attR following synapsis (Nash and Robertson, 1989).