The apocytochrome b gene in maize mitochondria does not contain introns and is preceded by a potential ribosome binding site

Abstract

The apocytochrome b (COB) gene has been isolated from maize (Zea mays L.) mitochondrial DNA. Sequence analysis reveals that the coding region of the gene is 1164 bp long and, in contrast with the homologous gene from yeast, does not contain introns or TGA (Trp) codons. The predicted polypeptide encoded by the gene has a mol. wt. of 42 868 daltons, and shows 48% amino acid sequence homology with the corresponding yeast and mammalian polypeptides. Hydropathic profiles of the polypeptide indicate the presence of nine, membrane spanning hydrophobic domains suggesting that it is organised in the inner mitochondrial membrane in a similar fashion to that proposed for apocytochrome b in other organisms. The COB gene is preceded by a sequence 5' -AGTTGTCA-3' which may act as a ribosome binding site in the mRNA since: (i) it shows 67.5% complementarity with an octanucleotide at the 3' end of the maize mitochondrial 18S rRNA, located in a position homologous to that of the Escherichia coli Shine and Dalgarno sequence, and (ii) a similar sequence precedes several other plant mitochondrial genes at a distance of 15-20 nucleotides from the ATG initiation codon. RNA transcript analysis shows that the gene is transcribed in a complex manner with the presumed mature mRNA ( 2.25 kb) probably being derived by sequential processing from a larger primary transcript.