A DNA Methyltransferase Homolog With a Chromodomain Exists in Multiple Polymorphic Forms in Arabidopsis

Chromodomains are thought to mediate protein-protein interactions between chromatin components. We have detected a chromodomain embedded within the catalytic region of a predicted Arabidopsis DNA methyltransferase that is diverged from other eukaryotic enzymes. The 791 residue “chromomethylase” (CMT1) is encoded by a floral transcript that is spliced from 20 exons and is present at only ~1/10−7 of total mRNA. Genomic sequencing reveals an ancient haplotype split at CMT1 between Col-0 + Metz and the other ecotypes examined. In the Col-0 + Metz haplotype, alternative mRNA processing at intron 13 truncates the coding region. In Ler, RLD, and No-0, similar truncation is caused by insertion of an intact retrotransposon, Evelknievel, which is present as a single copy in Ler and RLD and is currently methylated and inactive. Evelknievel is found at this site on a single branch that connects the Ler, RLD, and No-0 ecotypes but is absent from the genomes of all other ecotypes examined. A stop codon within exon 6 of the Metz ecotype confirms that CMT1 is nonessential. Nevertheless, comparison to CMT1 of Cardaminopsis arenosa, an outcrossing relative, indicates conservation for DNA methyltransferase function. We discuss how allelic diversity of CMT1 may reflect loosened selective constraints in a self-fertilizing species such as Arabidopsis thaliana.