Cloning and generation of a genetic map of bacteriophage N4 DNA.

Analysis of coliphage N4 development has been hindered by the lack of a genetic map. Conventional methods of complementation and recombination have been inadequate because N4 shows very high levels of recombination, which are independent of the host recombination system. We have cloned restriction fragments of the 72-kb genome of N4 and have used these clones to rescue a collection of suppressor-sensitive and temperature-sensitive mutants. After mutations were localized to a small region by marker rescue, complementation groups were defined. In this way several functions essential for transcription and replication have been mapped. Finally, a nonessential 6-kb region of the N4 genome has been identified by characterizing phage deletions isolated after heat and citrate treatment of virions.