The complete nucleotide sequence of cytochrome oxidase subunit II in maize mitochondria has been determined (Fox and Leaver, Cell 26:315-323, 1981). This gene, referred to as mox1, is entirely contained within a single 2.4 Kb EcoRI fragment of a total mitochondrial DNA digestion and is interrupted by a 794 base pair intron.

Using the dideoxy nucleotide chain termination sequence analysis of Sanger, a major portion of this same gene in Zea diploperennis has been sequenced. As in maize, the entire gene is contained within a single 2.4 Kb fragment when total mitochondrial DNA is digested with EcoRI. All major restriction endonuclease sites compared within this fragment were also found to be perfectly conserved between the two species. The following segments of mox1 have thus far been sequenced in Z. diploperennis:
 

Segment sequenced, and %	Nucleotide number
exon 1, 73%	116-432
intron, 100%	433-1227
exon 2, 53%	1228-1303, and 1467-1620

Comparisons of these sequences to those published for mox1 of maize have shown that they are identical. The 115 nucleotides of the 3' flanking region immediately following exon 2 have also been sequenced and compared. These sequences also share 100% homology with those in maize. The results are somewhat surprising considering the great differences between mitochondrial restriction patterns of maize and Z. diploperennis (Timothy et al., Maydica 28:139-144, 1983). DNA used for sequencing was isolated and cloned separately from two different sources of Z. diploperennis. These sources gave two very distinctive Z. diploperennis restriction patterns when digested with BamHI or EcoRI.

The sequence homologies between maize and Z. diploperennis indicate exceptional conservation. This was not wholly unexpected for the exon 1 and exon 2 segments of mox1. Slight departures from absolute homology of the introns and the 3' noncoding tails might be anticipated, but this was not the case. The most reasonable explanations for these results are low mutation rate and/or that these sequences play an important functional role.

The extreme conservation of sequences is indicative of close relationship between the taxa, even though the organization of the mitochondrial genomes may be distinct. This is consistent with our finding (Sederoff et al., PNAS 78:5953-5957, 1981) that while appreciable evolution of the mitochondrial genome in Zea may have occurred by major rearrangement of sequences, there is general sequence conservation.

R. E. Dewey, D. H. Timothy, C. S. Levings, III
 
 

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