Gene body DNA methylation in plants

• GbM is typified by mCG within coding regions, and depletion at the TSS and TTS.
• Species without gbM revealed an unexpected role for CMT3 in the evolution of gbM.
• Loss of gbM does not result in expected changes, which suggests dispensability.
• GbM might have evolved as a byproduct of transcriptional silencing of transposons.
• The function of gbM is enigmatic, but natural variation will aid in a discovery.

The type, amount, and location of DNA methylation within a gene provides pivotal information on the enzymatic pathway by which it was achieved and its functional consequences. In plants (angiosperms specifically), gene body methylation (gbM) refers to genes with an enrichment of CG DNA methylation within the transcribed regions and depletion at the transcriptional start and termination sites. GbM genes often compose the bulk of methylated genes within angiosperm genomes and are enriched for housekeeping functions. Contrary to the transcriptionally repressive effects of other chromatin modifications within gene bodies, gbM genes are constitutively expressed. GbM has intrigued researchers since its discovery, and much effort has been placed on identifying its functional role. Here, we highlight the recent findings on the evolutionary origin and molecular mechanism of gbM and synthesize studies describing the possible roles for this enigmatic epigenetic phenotype.

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