title: Atypical DNA methylation of genes encoding cysteine-rich peptides in Arabidopsis thaliana.
abstract: BACKGROUND
  In plants, transposons and non-protein-coding repeats are epigenetically silenced by CG and non-CG methylation. This pattern of methylation is mediated in part by small RNAs and two specialized RNA polymerases, termed Pol IV and Pol V, in a process called RNA-directed DNA methylation. By contrast, many protein-coding genes transcribed by Pol II contain in their gene bodies exclusively CG methylation that is independent of small RNAs and Pol IV/Pol V activities. It is unclear how the different methylation machineries distinguish between transposons and genes. Here we report on a group of atypical genes that display in their coding region a transposon-like methylation pattern, which is associated with gene silencing in sporophytic tissues.
  
  
  RESULTS
  We performed a methylation-sensitive amplification polymorphism analysis to search for targets of RNA-directed DNA methylation in Arabidopsis thaliana and identified several members of a gene family encoding cysteine-rich peptides (CRPs). In leaves, the CRP genes are silent and their coding regions contain dense, transposon-like methylation in CG, CHG and CHH contexts, which depends partly on the Pol IV/Pol V pathway and small RNAs. Methylation in the coding region is reduced, however, in the synergid cells of the female gametophyte, where the CRP genes are specifically expressed. Further demonstrating that expressed CRP genes lack gene body methylation, a CRP4-GFP fusion gene under the control of the constitutive 35 S promoter remains unmethylated in leaves and is transcribed to produce a translatable mRNA. By contrast, a CRP4-GFP fusion gene under the control of a CRP4 promoter fragment acquires CG and non-CG methylation in the CRP coding region in leaves similar to the silent endogenous CRP4 gene.
  
  
  CONCLUSIONS
  Unlike CG methylation in gene bodies, which does not dramatically affect Pol II transcription, combined CG and non-CG methylation in CRP coding regions is likely to contribute to gene silencing in leaves because loss of this methylation in synergid cells is associated with CRP gene expression. We discuss this unusual methylation pattern and its alteration in synergid cells as well as the possible retrogene origin and evolutionary significance of CRP genes that are methylated like transposons.
authors.affiliation: Gregor Mendel Institute of Molecular Plant Biology, Austrian Academy of Sciences, Vienna, Austria.
authors.forename: Wanhui
authors.initials: W
authors.lastname: You
authors.forename: Agata
authors.initials: A
authors.lastname: Tyczewska
authors.forename: Matthew
authors.initials: M
authors.lastname: Spencer
authors.forename: Lucia
authors.initials: L
authors.lastname: Daxinger
authors.forename: Marc W
authors.initials: MW
authors.lastname: Schmid
authors.forename: Ueli
authors.initials: U
authors.lastname: Grossniklaus
authors.forename: Stacey A
authors.initials: SA
authors.lastname: Simon
authors.forename: Blake C
authors.initials: BC
authors.lastname: Meyers
authors.forename: Antonius J M
authors.initials: AJ
authors.lastname: Matzke
authors.forename: Marjori
authors.initials: M
authors.lastname: Matzke
mesh_terms: D017360:Arabidopsis / Q000235:genetics*; D029681:Arabidopsis Proteins / Q000235:genetics*; D001483:Base Sequence; D003545:Cysteine / Q000235:genetics; D019175:DNA Methylation / Q000235:genetics*; D004251:DNA Transposable Elements / Q000235:genetics; D018744:DNA, Plant / Q000235:genetics; D012321:DNA-Directed RNA Polymerases / Q000235:genetics; D044127:Epigenesis, Genetic; D035264:Flowers / Q000235:genetics; D018506:Gene Expression Regulation, Plant / Q000235:genetics*; D020868:Gene Silencing; D008969:Molecular Sequence Data; D005810:Multigene Family; D009928:Organ Specificity; D010455:Peptides / Q000235:genetics; D018515:Plant Leaves / Q000235:genetics; D011110:Polymorphism, Genetic; D034622:RNA Interference; D018749:RNA, Plant / Q000235:genetics; D036226:Seedlings / Q000235:genetics; D017422:Sequence Analysis, DNA; D017423:Sequence Analysis, RNA
chemical_list: D029681:Arabidopsis Proteins; D004251:DNA Transposable Elements; D018744:DNA, Plant; D010455:Peptides; D018749:RNA, Plant; D012321:DNA-Directed RNA Polymerases; D003545:Cysteine