The effect of growth of Tetrahymena pyriformis in ethidium bromide (EthBr) on the structure and synthesis of mitochondrial DNA (mtDNA) has been investigated. During the first 5 h of growth in EthBr, mtDNA synthesis is inhibited 95% or more. After 10–15 h, this block is partially released and large numbers of replicating molecules accumulate, indicating that inhibition by EthBr primarily affects the rate of chain growth and not the initiation of new rounds of replication. The accumulated molecules sediment more rapidly than normal Tetrahymena mtDNA and do not contain enough single-strand regions to distinguish them from normal Tetrahymena mtDNA when banded in buoyant CsCl or NaI gradients. Electron microscopy shows that the predominant species in this rapidly sedimenting DNA is a linear molecule containing one symmetrical double-stranded replication loop of varying size located at its center. No degradation of mtDNA from cells grown in EthBr was detected in alkaline velocity gradients.

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