Inherited methylation patterns are guided by small RNAs
Probing further into the set of modifications on the DNA in plant pollen grains, Martienssen and colleagues decided to look at the particular set of chemical marks called methyl groups. When they separated out pollen grains in different stages of development they found distinct patterns of the attachment of methyl groups to DNA.
They also noticed the corresponding accumulation of small RNAs, including two classes of so-called short-interfering RNAs (siRNAs) tiny RNA molecules, 21 or 24 nucleotides in length -- involved in silencing gene expression. These small siRNAs act as guides to where methylation will occur, silencing gene expression.
Previous work by the Martienssen lab and their collaborators, including a team of pollen specialists from the Instituto Gulbenkian de Ciencia in Lisbon, Portugal, has shown that these epigenetic mechanisms are important for keeping transposons in check. Also known as "jumping genes" for their ability to be expressed and then re-insert themselves at random into a different area of the genome, transposons are dangerous because they can cause damage to DNA and disrupt genetic function.
In the current study, Martienssen's team discovered that while in sperm, some areas of DNA containing transposons had "lost" methyl groups, and thus had the potential to be expressed, the same stretches of DNA were observed to be methylated in the seed embryo. This was associated with the accumulation of 21 nucleotide long siRNA in the mature pollen and 24 nucelotide long siRNA in the seed embryo. Martienssen speculates that the loss of methylation in the sperm and subsequent r
|Contact: Edward Brydon|
Cold Spring Harbor Laboratory