The tight linkage between DNA replication and double-strand break repair in bacteriophage T4

Double-strand break (DSB) repair and DNA replication are tightly linked in the life cycle of bacteriophage T4, Indeed, the major mode of phage DNA rep lication depends on recombination proteins and can be stimulated by DSBs, D SB-stimulated DNA replication is dramatically demonstrated when T4 infects cells carrying two plasmids that share homology, A DSB on one plasmid trigg ered extensive replication of the second plasmid, providing a useful model for T4 recombination-dependent replication (RDR), This system also provides a view of DSB repair in T4-infected cells and revealed that the DSB repair products had been replicated in their entirety by the T4 replication machi nery. We analyzed the detailed structure of these products, which do not fi t the simple predictions of any of three models for DSB repair. We also pre sent evidence that the T4 RDR system functions to restart stalled or inacti vated replication forks. First, we review experiments involving antitumor d rug-stabilized topoisomerase cleavage complexes. The results suggest that f orks blocked at cleavage complexes are resolved by recombinational repair, likely involving RDR, Second, we show here that the presence of a T4 replic ation origin on one plasmid substantially stimulated recombination events b etween it and a homologous second plasmid that did not contain a T4 origin, Furthermore, replication of the second plasmid was increased when the firs t plasmid contained the T4 origin, Our interpretation is that origin-initia ted forks become inactivated at some frequency during replication of the fi rst plasmid and are then restarted via RDR on the second plasmid.