Deletion mutants of bacteriophage lambda : III. Physical structure of att

Deletion mutants and substitution mutants of bacteriophage λ have been used to examine the physical structure of the att^φ site in the λ chromosome which is essential for prophage integration. Integration-defective att mutants were analyzed by constructing heteroduplex DNA molecules containing one wild-type and one mutant strand and examining these heteroduplexes by electron microscopy. The results indicate that att^φ is less than 2500 base pairs in length and may be as small as 20 to 50 base pairs. Integration cross-overs between att^φ and its bacterial analog, att^B, occur within a region which is less than 12 base pairs in length. Moreover, there is no detectable base sequence homology between att^φ and att^B. These results suggest that λ integration is a truly unique system of recombination.