A critical base in the yeast mitochondrial nonanucleotide promoter. Abolition of promoter activity by mutation at the -2 position.

The yeast mitochondrial promoter comprises a highly conserved nonanucleotide sequence, 5'-ATATAAGTA-3'. Transcription is initiated both in vivo and in vitro at the 3'-terminal A (+1). The G at the -2 position is strictly conserved. The requirement for this nucleotide is established by comparing the in vitro promoter activity of nonanucleotides containing any of the four bases at the -2 position, obtained as synthetic oligonucleotides. Only the synthetic promoter having G at the -2 position exhibits any promoter activity.