12 DNA Topoisomerase-mediated Illegitimate Recombination

Abstract

Illegitimate recombination can be defined as a DNA rearrangement between nonhomologous and nonspecific sequences (Campbell 1962; Campbell 1971; Franklin 1971; Weisberg and Adhya 1977). It can generate deletions, duplications, insertions, substitutions, inversions, and transducing phage formation, and generally yields a novel junction at the site of crossover. This type of recombination is often found in bacteriophages, plasmids, and bacterial cells but is particularly frequent in mammalian somatic cells.

I. EXAMPLES OF ILLEGITIMATE RECOMBINATION
Deletions are the loss of a DNA segment from a chromosome. They are often found among spontaneous and induced mutations (Benzer 1961; Tessman 1962; Schwartz and Beckwith 1969). Deletion formation is usually independent of the key gene for homologous recombination, recA, in Escherichia coli (Franklin 1967; Inselburg 1967). A number of other E. coli genes involved in recombination and DNA repair—recB, recC, uvrA, uvrB, uvrC, lig, and endA—also do not seem to play a role in forming deletions (Anderson 1970; Spudich et al. 1970; see also Franklin 1971). Mutations in the DNA polymerase-I structural gene (polA) increase the frequency of deletions about 30-fold in the tonB-trp region of E. coli chromosome (Coukell and Yanofsky 1970).

Specialized transducing phages contain a portion of the bacterial genome that is contiguous to the lysogenic phage (Campbell 1962). For example, λdg transducing phages contain genes for galactose metabolism and are produced by abnormal excision of the λ prophage from the E. coli chromosome. This type of excision is independent of the phage int and xis site-specific recombination functions as...