Recombination hotspots in bacteriophage T4 are dependent on replication origins

Bacteriophage T4 recombination "hotspots" were first detected by the rescue of genetic markers from UV-irradiated phage particles. These hotspots have since been detected following treatments that yield other forms of DNA damage, and at least one is active in the absence of damage. The previous mapping of phage replication origins near the peaks of two recombination hotspots suggested that the origins cause the localized enhancement of recombination. Here we show that deletion of one origin eliminates the corresponding recombination hotspot, as judged by rescue of markers from UV-irradiated phage. Furthermore, insertion of either origin into a recombination "coldspot" enhances rescue of nearby markers. We conclude that these origins are necessary, and very likely sufficient, for the generation of recombination hotspots. We also show that the hotspots are active in the absence of both phage-encoded UvsY and host-encoded RecA proteins, suggesting that some of the stimulated recombination occurs by a synaptase-independent mechanism.