Asymmetric transcription of R plasmid NR1 in Proteus mirabilis.

The composite R plasmid NR1, its resistance transfer factor which specifies resistance to tetracycline (RTF-Tc component), and its r-determinants component were each denatured and centrifuged to equilibrium in CsCl density gradients containing polyuridylic acid-polyguanidylic acid. The complementary deoxyribonucleic acid strands of NR1 and the complementary strands of the RTF-Tc component could be separated by this technique because of a threefold difference in polyuridylic acid-polyguanidylic acid binding to the strands of the RTF-Tc component. The two strands of the r-determinants component bound equal amounts of polyuridylic acid-polyguanidylic acid. Hybridization of single strands of plasmid deoxyribonucleic acid with in vivo-labeled ribonucleic acid from Proteus mirabilis containing NR1 indicated that transcription within the RTF-Tc component is from the NR1 strand which preferentially binds polyuridylic acid-polyguanidylic acid, whereas transcription within the r-determinants component is predominantly from the complementary strand.