Academic journal article Genetics

Multiple-Copy Cluster-Type Organization and Evolution of Genes Encoding O-Methyltransferases in the Apple

Academic journal article Genetics

Multiple-Copy Cluster-Type Organization and Evolution of Genes Encoding O-Methyltransferases in the Apple

Article excerpt

ABSTRACT

Plant O-methyltransferases (OMTs) play important roles in secondary metabolism. Two clusters of genes coding for caffeic acid OMT (COMT) have been identified in the apple genome. Three genes from one cluster and two genes from another cluster were isolated. These five genes encoding COMT, designated Mdomt1-Mdomt5 (GenBank accession nos. DQ886018-DQ886022), were distinguished by a (CT)^sub n^ microsatellite in the 5'-UTR and two transposon-like sequences present in the promoter region and intron 1, respectively. The transposon-like sequence in intron 1 unambiguously traced the five Mdomt genes in the apple to a common ancestor. The ancestor must have undergone an initial duplication generating two progenitors, and this was followed by further duplication of these progenitors resulting in the two clusters identified in this study. The distal regions of the transposon-like sequences in promoter regions of Mdomt genes are capable of forming palindromic hairpin-like structures. The hairpin formation is likely responsible for nucleotide sequence differences observed in the promoter regions of these genes as it plays a destabilizing role in eukaryotic chromosomes. In addition, the possible mechanism of amplification of Mdomt genes in the apple genome is also discussed.

SECONDARY metabolites such as lignin, flavonoids, anthocyanins, suberin, and isoflavonoids are abundant in plants and they play various roles in plant growth and development as well as in plant interactions with the environment, including defense responses against microorganisms and herbivores (Schwab 2003). Plant O-methyltransferases (OMTs) play important roles in secondary metabolism, and many OMTs have been identified in plants. To date, the most thoroughly studied OMTs are caffeic acid OMT (COMT) and caffeoyl CoA OMT (CCOMT). While both are involved in lignin biosynthesis, COMTmethylates caffeic acid/5-hydroxyferulic acid, whereas, CCOMT methylates CoA ester (YE and VARNER 1995; LI et al. 1997; INOUE et al. 1998; MAURY et al. 1999). Flavonoid and isoflavone OMTs involved in the biosynthesis of phytoalexins have also been identified (CHRISTENSEN et al. 1998; HE et al. 1998). More recently, various OMTs involved in the biosynthetic pathways of floral scent components have been identified and characterized. Forexample,S-adenosyl-l-methionine(iso)eugenolOMT( IEMT),whichcatalyzeseugenolandisoeugenol to form volatile methyleugenol and isomethyleugenol, was isolated from Clarkia breweri (WANG et al. 1997). EugenolOMT( EOMT)andchavicolOMT(CVOMT),which convert eugenol and chavicol to methyleugenol and methylchavicol, respectively,havebeenidentifiedinOcimun basilicum (GANG et al. 2002). Also, two orcinol OMTs (OOMT),OOMT1 andOOMT2,havebeenisolatedfrom Rosa hybrida, and these efficiently methylate orcinol to produce 3,5-dimethoxytoluene (LAVID et al. 2002).Moreover, two OMTs involved in scent biosynthesis, RcOMT1 and RcOMT2, have also been isolated from R. chinensis (WU et al. 2003).

Although some OMT members have multiple functions (LI et al. 1997; GAUTHIER et al. 1998), most generally exhibit high substrate specificity (IBRAHIM et al. 1998). Recent studies have demonstrated that this substrate specificity can be altered by mutations of either a single or a few amino acid(s), thus suggesting that functionally distinct genes encodingOMTsmay have evolved from a common ancestral gene via duplication and mutation (GANG 2005). For example, although two genes encoding EOMT1 and CVOMT1 from O. basilicum share 90% amino acid identity, a single-amino-acid difference is responsible for the substrate discrimination between CVOMT1 and EOMT1 (GANG et al. 2002). This keyamino acid difference arose from a C-T transition, which is the most common form of observed DNA mutation. Therefore, CVOMT1 is likely to have evolved from EOMT1. Moreover, two genes encoding IEMT and COMT from C. breweri share 83% identity at the amino acid level, and their encoded proteins can be functionally interchangeable bymutually replacing seven amino acids (Wang and Pichersky 1998). …

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