Fractionation of the complementary strands of coliphage lambda DNA based on the asymmetric distribution of the poly I,G-binding sites.

Poly I,G interacts preferentially with one of the two complementary DNA strands of lambda and lambda-related phages 21, 434, and phi80, thus permitting preparative separation of the "dense" fraction, consisting of a complex between the intact strands C and poly I,G, from the less dense ("light") fraction containing the intact strands W, which bind 3-4 times less poly I,G. The isolated and self-annealed fractions are over 99% pure as far as their hybridization properties with complementary RNA are concerned. The interaction of poly I,G with intact and fragmented DNA of lambda and its deletion mutants could be interpreted as indicating the asymmetric distribution of poly I,G-binding deoxycytidine-(dC)-rich clusters between the strands of lambda DNA; on the left arm (55% G + C) the dC-clusters seem to be restricted to the C strand, whereas both complementary strands of the right arm (46% G + C) of lambda DNA contain these dC clusters. Thus, the two arms of lambda DNA differ not only in their average base composition (Hershey, 1966), but also in the mode of distribution of the dC-rich clusters, the latter possibly related to the initiation, termination and orientation of the DNA-to-RNA transcription from the complementary DNA strands.