The strands of DNA from lambda and related bacteriophages: isolation and characterization.

Each of the intact complementary strands of DNA from bacteriophage /\ has been isolated by virtue of the different buoyant densities they exhibit in alkaline C&l. Preparations of a given strand are free ( < 1%) of the other strand and do not contain detectable amounts of fragments. Individual &ran& exhibit no significant activity for the genes of X in the helper-phagge assay of Kaiser BE Hogness (1960), but renaturation of equimolar mixtures of the two isolated strands yields duplex molecules active for genes which span the /\ genome. The difference in density observed in alkaline C&l (Ap,& is not seen in neutral C&l and is presumed due to a difference between stranda in the sum of the frequencies of the titratable bases, G and T (AGT). This AGT has been estimated to involve about 1500 out of 50,000 base pairs in hDNA, and essentially all of these have been located in its left half. Comparison of these results with the binding of poly rG to the strands in whole and half molecules of h DNA indicate that in a given segment of this DNA the strand with the lesser GT content binds the greater amount of poly rG. The DNA’s from a hdg variant and from the X-related phage, 424, exhibit values of Apalk suf%lcient for the isolation of their strands. Four different types of 424 phage particles were observed on the basis of their buoyant densities in neutral CsCl. The most and least dense of these contain DNA that is 97 and 83% the size of X DNA, respectively, and the estimated number of base pairs responsible for the ACT in each is about 1900. Heteroduplex molecules can be formed between strands of DNA from X and the most dense 424 particle, providing the strand of highest CT content from one phage is combined with that of lowest GT content from the other. Such heteroduplexes exhibit densities in neutral CsCl midway between the densities of native and denatured DNA, indicating an extent of homology of about 60% between the two phages.