Methylation of a different kind

Abstract

In plants, cytosine methylation is important for gene silencing and maintenance of genome integrity, and is found in symmetrical CpG and CpNpG sites (where N represents any base) and at nonsymmetrical sites. Previous work established that methylation of CpG sites originates from DNA methyltransferase (MT), an enzyme known for its ten conserved domains. However, the enzyme does not seem to methylate CpNpG sites. Reduction in CpG methylation in Arabidopsis leads to developmental consequences, as shown in genetic backgrounds deficient in methylation. Until recently, it was unclear what enzyme methylated CpNpG sites and what role CpNpG methylation had in plant development.Three reports converged recently to identify chromomethylase (CMT) as necessary for CpNpG methylation. CMTs have similar domains to MTs, but contain an additional chromodomain, a motif found in several proteins involved in chromatin-based gene silencing. The groups took two separate strategies: Anders Lindroth et al. 1xRequirement of CHROMOMETHYLASE3 for maintenance of CpXpG methylation. Lindroth, A.M. et al. Science. 2001; 292: 2077–2080Crossref | PubMed | Scopus (432)See all References1 and Lisa Bartee et al. 2xArabidopsis cmt3 chromomethylase mutations block non-CG methylation and silencing of an endogenous gene. Bartee, L. et al. Genes Dev. 2001; 15: 1753–1758Crossref | PubMed | Scopus (240)See all References2 used forward genetics in Arabidopsis, whereas Charles Papa and colleagues 3xMaize chromomethylase Zea methyltransferase2 is required for CpNpG methylation. Papa, C.M. et al. Plant Cell. 2001; 13: 1919–1928PubMedSee all References3 used reverse genetics in maize. Lindroth et al. used a marker locus, SUPERMAN (SUP), previously shown to be a target sensitive to methylation changes 1xRequirement of CHROMOMETHYLASE3 for maintenance of CpXpG methylation. Lindroth, A.M. et al. Science. 2001; 292: 2077–2080Crossref | PubMed | Scopus (432)See all