Cilia were isolated from Tetrahymena pyriformis by an ethanol-calcium method. Addition of adenosine triphosphate to a suspension of intact or digitonin-extracted cilia caused a decrease of about 20 per cent in turbidity. Study of fractionated cilia showed that the presence of two distinct axonemal components, the outer fibers and the 30S dynein (the axonemal ATPase protein), was necessary for this effect on turbidity to occur. The decrease in turbidity is interpreted as a result of a specific interaction of ATP with these protein components causing an effective increase in hydration. The high nucleotide specificity suggests that the change in hydration is closely related to the processes responsible for motility. The outer fibers themselves swell when suspended in media of very low ionic strength. The concentration of salt needed to prevent this swelling (2 mM MgSO4 or 30 mM KCl) is about the same as that needed to keep dynein bound to the fibers. The recombination of purified 30S dynein with the outer fibers can be followed by the rise in turbidity resulting from increased dry mass of the particles.

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