Cytochrome Oxidase Subunit II Gene from Carrot Contains an Intron

Abstract

Introhs in the cytochrome oxidase subunit II (COXII) gene of plant mitochondrial DNA (mtDNA) have been observed only in monocots. The COXII genes in dicots investigated to date do not contain introns. This is the rirst report of an intron in the COXII gene of a dicot. The presence ofan intron in the carrot COXII intron was verified by restriction mapping and hybridization using specific maize and wheat COXII probes. Regions of the carrot COXII intron are homologous to the maize COXII intron and homologous to the wheat COXII intron-insert as demonstrated by hybridization. Homology of these regions was confirmed by sequencing portions of the gene. A comparison of the restriction map of the carrot COXII gene with the restriction maps of the COXII genes from pea, Oeaothera, maize, wheat, and rice revealed that the carrot map coincides with the rice restriction map.Cyt oxidase, an important part of the respiratory chain, is located in the inner mitochondrial membrane and directs the transfer of electrons from Cyt c to 02. The enzyme complex is composed of seven subunits. Three subunits (I, II, and III) are encoded by mitochondrial DNA, while the other four subunits (IV, V, VI, and VII) are encoded by nuclear DNA.The nucleotide sequence of the COXII gene from five plants, pea (12), Oenothera (8), maize (7), wheat (4), and rice (9), is highly conserved. Nucleotide homology of the COXII gene within monocots is 99%, within dicots is 89%, and between the two groups is 89%. The monocots, maize (7), wheat (4), rice (9), and rye (4), have introns in the COXII gene, while the dicots, pea (12), Oenothera (8), broad bean (4), and cucumber (4) do not contain introns. The COXII genes of all three monocots contain introns which begin at 391 bp;4 however, the introns vary in size (Fig. 1)

The Mitochondrial CoxII Intron has Been Lost in Two Different Lineages of Dicots and Altered in Others

“…During the analysis of these PCR products using Southern hybridization with synthetic oligonucleotides, the coxII intron was identified and characterized in the following dicot families: Apiaceae, Apocynaceae, Asclepiadaceae, and Solanaceae (Table 2). The present study substantiates the recent reports that the coxII gene from some dicots contains a ricelike intron (Ceci et al, 1987;Turano et al, 1987;Pruitt and Hanson, 1989;Jubier et al, 1990;Senda et al, 1991;Mann et al, 1991;De Bennedeto et al, 1992;Lippok, Brennicke, and Wissinger, 1992) in contrast to the report of Kao, Moon, and Wu (1984) who predicted that dicots lack a coxII intron.…”

“…Pruitt and Hanson (1989) also reported that the petunia intron inserts contain multiple insertions. On the other hand, Turano et al (1987) proposed that deletion of segments of the coxII intron could produce the same result. According to this 'deletion model', the coxII intron-inserts in wheat and rice coxII genes were precursors to the maize coxII intron, and the intron-inserts were lost either by alignment or recombination of the direct repeats.…”

“…Vigna has only one active coxII gene that is found in the nucleus. The coxII gene has been identified and sequenced from many different flowering plants including both monocots and dicots (Fox and Leaver, 1981;Hiesel and Brennicke, 1983;Bonen, Boer, and Gray, 1984;Kao, Moon, and Wu, 1984;Moon, Kao, and Wu, 1985;Grabau, 1987;Ceci et al, 1987;Turano et al, 1987;Pruitt and Hanson, 1989;Jubier et al, 1990;Mann et al, 1991;Senda et al, 1991;Lippok, Brennicke, and Wissinger, 1992). The coding region ofthe coxII gene in the monocots (Poaceae), maize, Zea mays, (Fox and Leaver, 1981); rice, Oryza sativa, (Kao, Moon, and Wu, 1984), and wheat, Triticum aestivum, (Bonen, Boer, and Gray, 1984) is interrupted by a single intron that varies from 794 bp to 1,265 bp in size.…”

“…The coxII gene in some dicots (i.e., evening primrose, Oenothera berteriana, in the Onagraceae [Hiesel and Brennicke, 1983]; pea, Pisum sativum, [Moon, Kao, and Wu, 1985], and soybean, Glycine max, [Grabau, 1987] in the Fabaceae) is continuous and completely lacks an intron. The presence of a coxII intron in a dicot was first reported by Turano et al (1987) in carrot (Daucus carota) from the Apiaceae. Recently, Lippok, Brennicke, and Wissinger (1992) found a second intron in the coxII gene in carrot.…”

Multiple trans-splicing events are required to produce a mature nad1 transcript in a plant mitochondrion.

“…A group II intron splits the coxlI gene in several, but not all, plant species that have been examined (Fox and Leaver 1981;Hiesel and Brennicke 1983;Turano et al 1987;Pruitt and Hanson 1989). The…”