ACTIVATION OF THE HUMAN MITOCHONDRIAL TRANSCRIPTION FACTOR A GENE BY NUCLEAR RESPIRATORY FACTORS - A POTENTIAL REGULATORY LINK BETWEEN NUCLEAR AND MITOCHONDRIAL GENE-EXPRESSION IN ORGANELLE BIOGENESIS

Mitochondrial transcription factor A (mtTFA), the product of a nuclear gene, stimulates transcription from the two divergent mitochondrial p romoters and is likely the principal activator of mitochondrial gene e xpression in vertebrates. Here we establish that the proximal promoter of the human mtTFA gene is highly dependent upon recognition sites fo r the nuclear respiratory factors, NRF-1 and NRF-2, for activity. Thes e factors have been previously implicated in the activation of numerou s nuclear genes that contribute to mitochondrial respiratory function. The affinity-purified factors from HeLa cells specifically bind to th e mtTFA NRF-1 and NRF-2 sites through guanine nucleotide contacts that are characteristic for each site. Mutations in these contacts elimina te NRF-1 and NRF-2 binding and also dramatically reduce promoter activ ity in transfected cells. Although both factors contribute, NRF-1 bind ing appears to be the major determinant of promoter function. This dep endence on NRF-1 activation is confirmed by in vitro transcription usi ng highly purified recombinant proteins that display the same binding specificities as the HeLa cell factors. The activation of the mtTFA pr omoter by both NRF-1 and NRF-2 therefore provides a link between the e xpression of nuclear and mitochondrial genes and suggests a mechanism for their coordinate regulation during organelle biogenesis.