Intact mitochondrial DNA (mtDNA) from mouse L cells has been introduced into E. coli by joining it to a small drug resistance plasmid, pSC101. The construction of the chimeric plasmids (pMM) makes it possible to assess the ability of bacterial cells to transcribe and translate a complete replicon of eukaryotic DNA. Highly amplified mtDNA setments from pMM plasmids will be used in in vitro transcription-translation experiments aimed at identifying the mtDNA gene products. Electron microscopy and S1 nuclease digestion of heteroduplexes between pMM DNA and mtDNA from other sources will reveal any sequence differences in mitochondrial genomes from normal and transformed cells. Plasmid DNA will also be used in attempts to replace mtDNA in L cells which have been depleted of endogenous mtDNA by an established procedure involving photosensitization by 5-bromodeoxyuridine.