Heteroduplexes of DNA Molecules of Lambdoid Phages: Physical Mapping of Their Base Sequence Relationships by Electron Microscopy

Abstract

The lambdoid phages are a group of related temperate coliphages. They share common cohesive ends, they undergo genetic recombination with each other to varying degrees, they are capable of genetic complementation to varying degrees, and they are serologically related.

In the work reported here, we have used the electron microscope heteroduplex method (Davis and Davidson, 1968; Westmoreland, Szybalski, and Ris, 1969; Davis, Simon, and Davidson, 1970) to map regions of sequence homology and nonhomology in the DNA molecules of λ, 434, 82, and 21. The first three were chosen because they are especially closely related. They all integrate between gal and bio in the Escherichia coli chromosome. Phage 21 is more distantly related to λ and integrates close to trp (Jacob and Wollman, 1961).

In the heteroduplex method, a mixture of two related DNAs, representing the DNA single strands by AA′ and BB′, is denatured and renatured. The renatured mixture contains the original homoduplex molecules, AA′ and BB′, and the heteroduplexes, AB′ and A′B. Under the conditions of mounting used here, both single- and double-stranded DNAs appear in the electron microscope as extended filaments, but the single strands are usually perceptibly thinner and more kinky than the double strands. Regions of homology in a heteroduplex appear double-stranded; regions of nonhomology show two single strands.

EXPERIMENTAL METHODS
The mutant λcI26 and the lysogenic strains of E. coli C600(434), C600(82), and C600(21) were obtained from the stocks of Dr. Jean Weigle. The 434gal was isolated, grown, and purified by J. S. Parkinson.