Alignment of two DNA helices: a model for recognition of DNA base sequences by the termini-generating enzymes of phage lambda, 186, and P2.

Based on the 3'- and 5'-terminal sequences of DNA of phage lambda, P2, and 186, a model is proposed for recognition of DNA sequences by enzymes responsible for generation of cohesive ends. Two copies of the cohered ends, either on separate molecules or on a concatemer, are aligned with their helical axes parallel but running in opposite directions. The nicking system is dimeric, with each of the two monomers carrying identical sequence-recognition sites. Two pairs of nicks are introduced into the two aligned DNA molecules by the nicking system. The applicability of this model to other biological processes, such as integration of a viral genome into a host genome and the cutting of concatemeric T7 DNA, is discussed.