Chromatin and siRNA pathways cooperate to maintain DNA methylation of small transposable elements in Arabidopsis.
BACKGROUND
DNA methylation occurs at preferred sites in eukaryotes. In Arabidopsis, DNA cytosine methylation is maintained by three subfamilies of methyltransferases with distinct substrate specificities and different modes of action. Targeting of cytosine methylation at selected loci has been found to sometimes involve histone H3 methylation and small interfering (si)RNAs. However, the relationship between different cytosine methylation pathways and their preferred targets is not known.


RESULTS
We used a microarray-based profiling method to explore the involvement of Arabidopsis CMT3 and DRM DNA methyltransferases, a histone H3 lysine-9 methyltransferase (KYP) and an Argonaute-related siRNA silencing component (AGO4) in methylating target loci. We found that KYP targets are also CMT3 targets, suggesting that histone methylation maintains CNG methylation genome-wide. CMT3 and KYP targets show similar proximal distributions that correspond to the overall distribution of transposable elements of all types, whereas DRM targets are distributed more distally along the chromosome. We find an inverse relationship between element size and loss of methylation in ago4 and drm mutants.


CONCLUSIONS
We conclude that the targets of both DNA methylation and histone H3K9 methylation pathways are transposable elements genome-wide, irrespective of element type and position. Our findings also suggest that RNA-directed DNA methylation is required to silence isolated elements that may be too small to be maintained in a silent state by a chromatin-based mechanism alone. Thus, parallel pathways would be needed to maintain silencing of transposable elements.
Tran
Robert K
RK
Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, WA 98109, USA. rtran@fhcrc.org
Zilberman
Daniel
D
de Bustos
Cecilia
C
Ditt
Renata F
RF
Henikoff
Jorja G
JG
Lindroth
Anders M
AM
Delrow
Jeffrey
J
Boyle
Tom
T
Kwong
Samson
S
Bryson
Terri D
TD
Jacobsen
Steven E
SE
Henikoff
Steven
S
D017360:Arabidopsis / Q000235:genetics* / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D060565:Argonaute Proteins; D002843:Chromatin / Q000378:metabolism*; D019175:DNA Methylation; D004251:DNA Transposable Elements / Q000235:genetics* / Q000502:physiology; D015257:DNA-Cytosine Methylases / Q000235:genetics / Q000502:physiology; D020869:Gene Expression Profiling; D011495:Histone-Lysine N-Methyltransferase / Q000235:genetics / Q000502:physiology; D008780:Methyltransferases / Q000235:genetics / Q000502:physiology; D020411:Oligonucleotide Array Sequence Analysis; D034741:RNA, Small Interfering / Q000502:physiology*
C471209:AGO4 protein, Arabidopsis; D029681:Arabidopsis Proteins; D060565:Argonaute Proteins; D002843:Chromatin; D004251:DNA Transposable Elements; D034741:RNA, Small Interfering; D015257:DNA-Cytosine Methylases; D008780:Methyltransferases; C429766:CMT3 protein, Arabidopsis; C470731:DRM1 protein, Arabidopsis; C470732:DRM2 protein, Arabidopsis; D011495:Histone-Lysine N-Methyltransferase; C471228:SUVH4 protein, Arabidopsis