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Site-specific Recombination in Phage Lambda

Robert A. Weisberg, Arthur Landy


Cells lysogenic for λ carry a quiescent form of the viral chromosome called prophage. The prophage differs from the DNA of viral particles in two important ways: (1) The ends of the prophage and of the viral particle DNA are at different points in the nucleotide sequence, and (2) the prophage ends are covalently joined to host DNA. Campbell (1962) proposed that viral particle DNA is converted to prophage by the joining of the ends followed by insertion of the resulting circular molecule into the host DNA. Insertion occurs by reciprocal recombination at specific sites (attachment or att sites) in each chromosome (Fig. 1). This proposal has received extensive experimental support and, indeed, was generally accepted when the first edition of this book was written (see Gottesman and Weisberg 1971).

A stable lysogen is formed when an infecting viral particle succeeds both in repressing lytic functions and in inserting its DNA. The inserted prophage is then replicated as part of the bacterial chromosome. In the rare event that repression is not followed by insertion (abortive lysogeny), the viral chromosome cannot replicate and so is lost by dilution as the host cell divides. Excision of the prophage from the chromosome is rare during normal bacterial growth but occurs readily following repressor inactivation. If the repressor is inactivated only briefly (abortive or transient derepression), prophage excision can occur without lytic growth. The excised DNA is then frequently lost as the cell divides (prophage curing).

Intricate controls ensure that insertion occurs only...

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