Specific requirement for ATP at an early step of in vitro transcription of human mitochondrial DNA.

The ATP concentrations allowing transcription of both heavy- and light-strand of human mtDNA in a HeLa cell mitochondrial lysate were found to cover a broad range, with a maximum around 2.5 mM, and with reproducible differences in the ATP response curves for the two transcription events. Direct measurements showed that nonspecific ATP degradation during the assay did not account for the high ATP requirement. 5'-Adenylyl imidodiphosphate (p[NH]ppA), an ATP analog with a nonhydrolyzable beta-gamma bond, was unable to substitute for ATP in supporting mtDNA transcription but greatly stimulated this transcription in the presence of a low concentration of exogenous ATP. Evidence was obtained indicating that p[NH]ppA did not support an early event in mtDNA transcription (formation of preinitiation complex or initiation), whereas this analog could substitute effectively for ATP in the subsequent elongation steps. These results pointed to a specific requirement for ATP at an early step of the transcription process.