SUMMARY: μ particles of stock 540 (syngen 1) of were isolated and studied. Electron microscopy showed that isolated μ particles were very similar to bacteria in fine structure, the general appearance being similar to that found by earlier workers. Contamination of the preparations with other cell particles and with the paramecium food bacteria was low. The isolated μ particles contained DNA, RNA, protein and probably carbohydrate and lipid. The DNA of the particles was characterized by density-gradient centrifugation in CsCl and by determination of Tm. The density of the DNA was estimated to be 1·694 g./c.c., a value very similar to that for the DNA of 540 macronuclei, which was estimated to be 1·693 g./c.c. The Tm was likewise similar, being 82·0°, against 81·8° for Paramecium macronuclear DNA. However, this DNA was associated with the μ particle, as no such DNA could be prepared from genotypically identical which did not possess μ particles, and the DNA associated with the μ particles was not susceptible to DNase action on treatment of isolated μ particles with the enzyme before DNA extraction. Hence it seems that the μ particle has a membrane-protected DNA of a similar buoyant density to the DNA. Isolated μ particles possessed an active DNA-dependent system capable of the incorporation of ribonucleoside triphosphates into an acid-insoluble product. The reaction needed the presence of ATP, GTP, and UTP for optimal activity, and had an absolute dependence on Mg. The reaction was inhibited by actinomycin D, and a ribonuclease-sensitive product was synthesized.


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