TEMs of Kleinschmidt preparations showing: (a) circular DNA of OX 174RF 21S; (b) molecular complexes resulting from the reaction of this DNA with E. coli RNA polymerase upon addition of the four regular nucleoside triphosphates: ATP, CTP, GTP, UTP. After fifteen minutes at room temperature, characteristic convolutes formed by filaments of varying widths are found in close association with the DNA-enzyme complex.