Wednesday, September 13, 2006
Nearly a century ago, two geneticists described 'rogue' pea plants with an unorthodox pattern of inheritance. William Bateson and Caroline Pellew found that crossing inferior rogues with normal plants always produced rogue offspring, suggesting that the rogue appearance was a dominant trait. The real surprise came when rogue progeny were crossed back to normal plants. Following the principles of Mendelian inheritance, these crosses should have produced a mix of normal and rogue plants, but they produced only rogue plants. The phenomenon, later dubbed 'paramutation', allowed the rogues to break the rules by acting 'epigenetically' - inducing heritable changes in gene expression without DNA mutations. In one-sided interactions between gene pairs, or alleles, only 'paramutagenic' alleles can attenuate, and eventually silence, the expression of 'paramutable' alleles.
Epigenetic silencing involves chemical modifications to DNA and the histone proteins that remodel the chromatin surrounding DNA, rendering genes inaccessible to transcription-related proteins. Epigenetic silencing also targets 'transposons', genetic elements that can jump around the genome. Both paramutagenic alleles and transposons contain tandem or inverted repetitive DNA sequences. Recent work in a variety of species suggests that such repeats can induce heritable silencing when they trigger the production of double-stranded RNAs, which are then processed into small interfering RNAs that can inactivate genes through DNA methylation and other mechanisms. [RxPG News RNA]
Based on the PLoS Biology open access paper "Initiation, Establishment, and Maintenance of Heritable MuDR Transposon Silencing in Maize Are Mediated by Distinct Factors".
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