The topography of lambda DNA: Polyriboguanylic acid binding sites and base composition

Abstract

The positions of the ten poly(rG) binding sites on the two strands of λ DNA have been determined with the aid of the families of overlapping DNA fragments described in the preceding paper and the DNA of deletion mutants. Nine sites are on the r-strand ‡ ‡ Terminology.The right and left of linear DNA correspond to the right and left of the linkage map for vegetative (Hogness and Simmons,1964).The (IT)r(/IT)-strand of DNA is transcribed rightward, having its 5 end to the right.The l-strand is its complement. : two are located in the right half, 0·84 ± 0·03 and 0·91 ± 0·02 molecular length unit from the left end; and seven are evenly distributed within 0·4 unit from the left end. The tenth site is on the l-strand, 0·60 ± 0·03 unit from the left end. Another site was mapped on the r-strand of the bio segment of Escherichia coli DNA found in λ bio variants; the l-strand of this segment contains no sites. All λ sites are alike in binding the same amount of poly(rG) and are considered to be derived from a set of repeated sequences with two orientations: gr sequences generate sites on r-strands; gl sequences, on l-strands. Without exception, this orientation corresponds to the orientation of transcription at the location of the sequence, transcription being from the r-strand at gr sites and from the l-strand at the gl site. The location of these g sequences indicates that they are not equivalent to operators, promoters or rho-dependent termination sites. However, g sequences do exhibit a positive topographic correlation with the multiple mRNA molecules derived from single transcriptons in λ. This correlation suggests that g sequences determine where the mRNA from a single transcripton is to be divided. The selective advantages that could result from this division and the likely mechanisms for effecting it are discussed. The base composition of λ DNA as a function of molecular length has been determined from the buoyant density of the overlapping fragments, complementing and extending previous maps.