Partial Sequence of Gene for Cytochrome Oxydase Subunit II of Sunflower Mitochondrial DNA

The mitochondrial DNA (mtDNA) in higher plants is larger than that of animals and fungi for 1-2 orders of magnitude (1). Despite its unusual complexity its informational content is very similar to that of other eukaryote organisms studied so far and it includes genes for 26S, 18S, and 5S ribosomal RNAs and tRNAs and for some proteins which are components of respiratory chain and F1-Fo ATP--ase complex (2). Among these the gene for cytochrome oxydase subunit II (COXII gene) has been sequenced both in monocotyledonous (3-5) and dicotyledonous plants (6-7). The comparative analysis of the organization of COXII genes in the five systems studied so far reveals interesting features concerning coding, non coding and putative regulatory regions of the genes: in monocot plants (maize (3), wheat (4), and rice (5)) an intron of various length interrupts the coding region at exactly the same position; the coding sequences are highly conserved (nucleotide homology is higher than 88%), in contrast with animal mitochondrial genes which evolved more rapidly; common regions of the introns, characteristic of maize, wheat and rice, show also high level of homology (98%) suggesting a possible structural or functional role for the untranslated region of the gene; 5' non coding regions appear to be highly homologous only in the comparison rice/wheat (97%); 3' non coding sequences are well conserved among monocot genes (90%) but very different from those of pea and oenothera; the codon usage is quite similar to the universal code, only the CGG codon has probably a different meaning: triptophane vs arginine.