FATE OF EXOGENOUS BACTERIAL DEOXYRIBONUCLEIC ACIDS IN BARLEY SEEDLINGS

Heteropycnic labelled bacterial DNA's (DNA's of different density) are taken up by barley root cells and less rapidly by the shoot. Some of these molecules combine with the endogenous DNA (d = 1.702 g/cm3) whereas some are rejected. For example, when one-day old seedlings are treated with Micrococcus lysodeikticus [32P]DNA (d = 1.731 g/cm3), radioactive DNA molecules are found at density 1.712 g/cm3. These molecules yield, on sonication, labelled DNA of about 1.731 g/cm3. One-day old barley seedlings, incubated with heteropycnic unlabelled bacterial DNA's, incorporate [3H]thymidine into DNA molecules of intermediate density if more than 24 hours have passed after treatment with DNA. This phenomenon is more pronounced in root cells than in the shoot. Barley incorporates [3H]thymidine into molecules of intermediate density (d = 1.712 g/cm3), if incubated with DNA from M. lysodeikticus (d = 1.731 g/cm3). However, in normal barley DNA (d = 1.702 g/cm3), the incorporation of [3H]thymidine appears to be depressed. The newly formed heavy molecules can be reversibly denatured by heat into molecules of density 1.727 g/cm3. They yield, on sonication, molecules with densities of 1.731 g/cm3 and 1.702 g/cm3. These two types of DNA can be denatured further into molecules of density 1.742 and 1.717 g/cm3, respectively. If incubation is carried out with denatured or ultrasonicated M. lysodeikticus DNA, with barley DNA, or with water, only the normal endogenous DNA appears labelled after treatment with [3H]thymidine. A model is proposed according to which the double strand of M. lysodeikticus DNA is joined end-to-end with barley DNA and the new structure is subsequently replicated. Some implications of this model are discussed. © 1969.