The synthesis of mitochondrial DNA (mDNA) in HeLa cells is selectively inhibited by relatively low concentrations of ethidium bromide . After exposure of cells to strongly inhibitory concentrations of the drug, the apparent superhelix density of mDNA is rapidly increased, as judged by its buoyant density in CsC1 in the presence of ethidium bromide . Mitochondrial DNA synthesized in the presence of partially inhibitory concentrations of ethidium bromide is also altered in its buoyant density in the presence of the dye, but is more heterogeneous in this respect . However, the change in buoyant density of newly synthesized mDNA may be explained by changes in structure other than a change in superhelix density, as indicated by its increased resistance to digestion by pancreatic DNase .

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The mechanism of inhibition of host cell protein synthesis by poliovirus has been studied by examining the behavior of host polyribosomes and polyribosome-associated messenger ribonucleic acid (mRNA). Virus infection appears to result in a specific inhibition of the initiation of translation of host cell mRNA. The site of inhibition does not appear to be mRNA itself. Human cells respond to virus infection by producing a factor antagonistic to the virus inhibition which promotes the initiation of host mRNA translation. The production of the host factor is sensitive to actinomycin; however, the behavior of the host cell factor and that of host mRNA appear distinctly different.