RESUMO
Analogous to genetically distinct alleles, epialleles represent heritable states of different gene expression from sequence-identical genes. Alleles and epialleles both contribute to phenotypic heterogeneity. While alleles originate from mutation and recombination, the source of epialleles is less well understood. We analyze active and inactive epialleles that were found at a transgenic insert with a selectable marker gene in Arabidopsis. Both converse expression states are stably transmitted to progeny. The silent epiallele was previously shown to change its state upon loss-of-function of trans-acting regulators and drug treatments. We analyzed the composition of the epialleles, their chromatin features, their nuclear localization, transcripts, and homologous small RNA. After mutagenesis by T-DNA transformation of plants carrying the silent epiallele, we found new active alleles. These switches were associated with different, larger or smaller, and non-overlapping deletions or rearrangements in the 3' regions of the epiallele. These cis-mutations caused different degrees of gene expression stability depending on the nature of the sequence alteration, the consequences for transcription and transcripts, and the resulting chromatin organization upstream. This illustrates a tight dependence of epigenetic regulation on local structures and indicates that sequence alterations can cause epigenetic changes at some distance in regions not directly affected by the mutation. Similar effects may also be involved in gene expression and chromatin changes in the vicinity of transposon insertions or excisions, recombination events, or DNA repair processes and could contribute to the origin of new epialleles.
Assuntos
Arabidopsis/genética , Cromatina/metabolismo , Epigênese Genética , Rearranjo Gênico , Regiões 3' não Traduzidas/genética , Alquil e Aril Transferases/genética , Alquil e Aril Transferases/metabolismo , Alelos , Proteínas de Arabidopsis/genética , Proteínas de Arabidopsis/metabolismo , Cromatina/genética , Imunoprecipitação da Cromatina , Metilação de DNA , DNA Bacteriano/genética , Regulação da Expressão Gênica de Plantas , Histona-Lisina N-Metiltransferase/genética , Mutagênese , Mutação , Fenótipo , Plantas Geneticamente Modificadas/genética , Recombinação Genética/genética , Deleção de Sequência/genética , Ativação Transcricional/genéticaRESUMO
Epigenetic changes of gene expression can potentially be reversed by developmental programs, genetic manipulation, or pharmacological interference. However, a case of transcriptional gene silencing, originally observed in tetraploid Arabidopsis thaliana plants, created an epiallele resistant to many mutations or inhibitor treatments that activate many other suppressed genes. This raised the question about the molecular basis of this extreme stability. A combination of forward and reverse genetics and drug application provides evidence for an epigenetic double lock that is only alleviated upon the simultaneous removal of both DNA methylation and histone methylation. Therefore, the cooperation of multiple chromatin modifications can generate unanticipated stability of epigenetic states and contributes to heritable diversity of gene expression patterns.
