Nucleic acids of , Pc, a small euryhaline hymenostome ciliate isolated from planktonic debris off the coast of Florida, were isolated and partially characterized. DNA was purified from macronuclear preparations and the final product was fibrous, contained less than 1% protein and was essentially free of carbohydrate and RNA. The base composition, determined chromatographically, was (mol% ± ): guanine, 10.8 ± 1.3; adenine, 41.5 ± 1.2; cytosine, 9.0 ± 0.6; thymine, 38.7 ± 1.7. The average molecular weight based on estimates made from sedimentation rate constants determined in sucrose gradients was about 3 × 10. The DNA exhibited a sharp thermal transition over a narrow temperature range accompanied by an increase in absorbance of 42% at 260 nm. The midpoint of thermal transition, T, was 77.6 °C, corresponding to a guanine + cytosine content of 20.2 mol%, a result supported by the chromatographic studies and among the lowest ever reported for a ciliate. Reassociation of heat-denatured preparations in 1 -Na, pH 7.0 at T − 25 °C followed second-order kinetics for about 30% of the reaction and revealed the presence of two components, a slowly reassociating main component comprising about 96% of the DNA and a rapidly reassociating minor component comprising about 4% of the DNA. The molecular complexity of the main component was 2.4 × 10 and that of the minor component was about 10. Sucrose gradient analysis of several purified RNA preparations showed three components exhibiting sedimentation coefficients of 24.8 ± 1.2, 17.4 ± 1.0 and 4.0 ± 0.8 S. These values were comparable with those obtained for RNA of the freshwater ciliates and .


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