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The Stability of Int and Xis Functions

Robert A. Weisberg, Max E. Gottesman


In the past few years the steps by which bacteriophage λ inserts itself into the bacterial chromosome have become increasingly clear (see Chapter 6). The first steps include the circularization of the phage chromosome and the transcription and translation of the int gene. Int is a site specific recombination function which recognizes a special nucleotide sequence on the phage chromosome and another nucleotide sequence on the bacterial chromosome. Integration results from a recombination between phage and bacterial chromosomes which occurs within the two specific sequences recognized by Int (see Fig. 1 and Chapter 6).

In order to facilitate discussion of Int-promoted recombination, we shall adopt the following nomenclature: P.P′ and B.B′ represent the Int-specific nucleotide sequences which constitute the attachment sites (att) on the phage and bacterial chromosomes, respectively. The dot represents the site of the Int-promoted crossover. Thus, recombination between P.P′ and B.B′ results in two new attachment sites in which the elements on either side of the dot are interchanged: P.B′ and B.P′. Both of these new att’s are also responsive to Int and will recombine with each other to regenerate the original P.P′ and B.B′ (see Fig. 1). The reverse recombination, P.B′ × B.P′, is unique because it has an additional requirement which is not shared by the original pair nor by any other combination of attachment sites; it does not proceed in the absence of a second phage specific function, Xis, which depends on the xis gene (Guarneros and Echols, 1970).

In order to gain some...

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