Howden Africa Ltd, South Africa

Abstract

The members of a set of overlapping fragments of λ DNA have been isolated. The set is divided into two groups, the right and left families. All members of the right family contain the cohesive site at the right end of λ DNA; members of the left family contain the cohesive site at the left end. The members of each family differ in molecular length and this characteristic was used to order and to isolate the fragments. Each family is comparable to an ordered set of overlapping deletions and the principles of deletion mapping can therefore be applied to both chemically and genetically defined characteristics of λ DNA.Mapping of mutant loci is possible because (1) all fragments in a family are active in the Kaiser-Hogness helper phage assay for genetic activity, and (2) the distance between a given locus and that end of λ DNA denning the family is equal to the length of the shortest fragment which contains the locus. The lengths of such fragments were determined from their sedimentation coefficients to yield the following positions for six amber mutations, expressed as fR, the fraction of the length of whole λ DNA measured from the right end: N7, 0·268; O29, 0·214; P80, 0·186; Q21, 0·090; R54, 0·55; R221, 0·052. The left boundary of the immλimm434 non-homology region was mapped at fR= 0·252.The advantage of the mapping methods described here is that they are applicable to point mutations and to characteristics which can be defined by direct chemical analysis of the DNA. In the following paper (Champoux & Hogness, 1972), the position and orientation of the ten specific sequences which generate poly(rG) binding sites and the topography of base composition in λ DNA were determined.