The process of prophage integration by phage λ and thefunction and structure of the chromosomal elements required for λintegration have been studied with the use of λ deletion mutants.Since attφ, the substrate of the integration enzymes, is not essentialfor λ growth, and since attφ resides in a portion of the λ chromosomewhich is not necessary for vegetative growth, viable λ deletionmutants were isolated and examined to dissect the structure of attφ.
Deletion mutants were selected from wild type populations bytreating the phage under conditions where phage are inactivated at arate dependent on the DNA content of the particles. A number ofdeletion mutants were obtained in this way, and many of these mutantsproved to have defects in integration. These defects were defined byanalyzing the properties of Int-promoted recombination in these attmutants.
The types of mutants found and their properties indicated thatattφ has three components: a cross-over point which is bordered oneither side by recognition elements whose sequence is specificallyrequired for normal integration. The interactions of the recognitionelements in Int-promoted recombination between att mutants wasexamined and proved to be quite complex. In general, however, itappears that the λ integration system can function with a diversearray of mutant att sites.
The structure of attφ was examined by comparing the geneticproperties of various att mutants with their location in the λ chromosome.To map these mutants, the techniques of heteroduplex DNAformation and electron microscopy were employed. It was found thatintegration cross-overs occur at only one point in attφ and that therecognition sequences that direct the integration enzymes to theirsite of action are quite small, less than 2000 nucleotides each.Furthermore, no base pair homology was detected between attφand its bacterial analog, attB. This result clearly demonstratesthat λ integration can occur between chromosomes which have little,if any, homology. In this respect, λ integration is unique as asystem of recombination since most forms of generalized recombinationrequire extensive base pair homology.
An additional study on the genetic and physical distances in theleft arm of the λ genome was described. Here, a large number ofconditional lethal nonsense mutants were isolated and mapped, anda genetic map of the entire left arm, comprising a total of 18 genes,was constructed. Four of these genes were discovered in this study.A series of λdg transducing phages was mapped by heteroduplexelectron microscopy and the relationship between physical andgenetic distances in the left arm was determined. The resultsindicate that recombination frequency in the left arm is an accuratereflection of physical distances, and moreover, there do not appearto be any undiscovered genes in this segment of the genome.
