Active DNA demethylation in plant companion cells reinforces transposon methylation in gametes.

The Arabidopsis thaliana central cell, the companion cell of the egg, undergoes DNA demethylation before fertilization, but the targeting preferences, mechanism, and biological significance of this process remain unclear. Here, we show that active DNA demethylation mediated by the DEMETER DNA glycosylase accounts for all of the demethylation in the central cell and preferentially targets small, AT-rich, and nucleosome-depleted euchromatic transposable elements. The vegetative cell, the companion cell of sperm, also undergoes DEMETER-dependent demethylation of similar sequences, and lack of DEMETER in vegetative cells causes reduced small RNA-directed DNA methylation of transposons in sperm.

Genome-wide demethylation of Arabidopsis endosperm.

Parent-of-origin-specific (imprinted) gene expression is regulated in Arabidopsis thaliana endosperm by cytosine demethylation of the maternal genome mediated by the DNA glycosylase DEMETER, but the extent of the methylation changes is not known. Here, we show that virtually the entire endosperm genome is demethylated, coupled with extensive local non-CG hypermethylation of small interfering RNA-targeted sequences. Mutation of DEMETER partially restores endosperm CG methylation to levels found in other tissues, indicating that CG demethylation is specific to maternal sequences.

Evolving potassium channels by means of yeast selection reveals structural elements important for selectivity.

Potassium channels are widely distributed. To serve their physiological functions, such as neuronal signaling, control of insulin release, and regulation of heart rate and blood flow, it is essential that K+ channels allow K+ but not the smaller and more abundant Na+ ions to go through. The narrowest part of the channel pore, the selectivity filter formed by backbone carbonyls of the GYG-containing K+ channel signature sequence, approximates the hydration shell of K+ ions.