Tandem Genetic Duplications in a Derivative of Phage Lambda

Gene duplication followed by independent evolution of the duplicated genes appears to be a basic feature of the genetics of higher organisms (see Bryson and Vogel, 1965). Such duplications are rare in Escherichia coli as judged by measurements of repeating DNA sequences (Britten and Kohne, 1968), although there are multiple copies of the ribosomal RNA genes in E. coli (Yankofsky and Spiegelman, 1962; Moore and McCarthy, 1967). There have been few studies of tandem duplications in phages. Duplications in the T4 r II region have been obtained by a strong selection technique (Weil et al., 1965; Parma and Ingraham, 1970). We have found that many of the spontaneous “revertants” of the deletion mutant t del 33 (Franklin, 1967) are duplication mutants, with changes at different locations on the phage chromosome. This phage has many advantages for studying the properties and mechanism of formation of duplication mutants. Some advantages which we have exploited in the present work are as follows. (1) Since the base sequence in 80- λ hy 1 DNA is nonpermuted and the phage chromosome can be extracted as a DNA molecule of definite length, a duplication mutant can be identified as such and the end points of the duplicated segment can be located by electron microscopy of DNA heteroduplexes. (2) The dependence of phage buoyant density on DNA length provides a nonspecific selection method for mutants which have longer DNA molecules. (Selection by plaque morphology is also possible in many cases.) (3) An independent method of identifying duplication mutants is based...