Characterization of a DNA-binding protein implicated in transcription in wheat mitochondria

To investigate the transcriptional apparatus in wheat mitochondria, mitochondrial extracts were subjected to column chromatography and protein fractions were analyzed by in vitro transcription and mobility shift assays. Fractions eluting from DEAE-Sephacel between 0.2 and 0.3 M KCl displayed DNA-binding activity and supported specific transcription initiated from a wheat cox2 promoter. The active DEAE-Sephacel pool was further resolved by chromatography on phosphocellulose. Fractions that exhibited DNA-binding activity and that stimulated both specific and nonspecific transcription in vitro were highly enriched in a 63-kDa protein (p63). From peptide sequence obtained from purified p63, a cDNA encoding the protein was assembled. The predicted amino acid sequence (612 amino acid residues, 69 kDa) contains a basic N-terminal targeting sequence expected to direct transport of the protein into mitochondria. The p63 sequence also features an acidic domain characteristic of transcriptional activation factors, as well as sequence blocks displaying limited similarity to positionally equivalent regions in sigma factors from eubacteria related to mitochondria. Recombinant p63 possesses DNA-binding activity, exhibiting an affinity for the core cox2 promoter element and upstream regions in gel shift assays and having the ability to enhance specific transcription in vitro. Transcripts encoding p63 are expressed at an early stage in the germination of isolated wheat embryos, in a temporal pattern parallelling that of newly synthesized precursors of cox2, a mitochondrial gene. Taken together, these data suggest a role for p63 in transcription in wheat mitochondria.