Transgenic Epigenetics: Using Transgenic Organisms to Examine Epigenetic Phenomena

Figure 2

Paramutation at the maize b1 locus. The two alleles that participate in paramutation at the b1 locus are identical in sequence and contain an identical control region consisting of seven tandem repeats (red and white boxes). However, the B-I allele is highly transcribed while the B′ allele is not. The two alleles exhibit epigenetic differences in chromatin structure, histone modifications, and DNA methylation and may be associated with distinct proteins that maintain these epigenetic states. The tandem repeats are bidirectionally transcribed in both B-I and B′ plants, producing repeat RNA that then forms dsRNA and is processed into siRNAs. The proteins MOP1, RMR6, and MOP2 are important for the production and amplification of the dsRNA and siRNAs. The siRNAs are hypothesized to direct chromatin modifications at the tandem repeats via mechanisms and proteins that are currently unknown, but this process is blocked at the B-I allele, potentially by the active chromatin state, bound proteins, or nuclear environment. Paramutation occurs in heterozygous plants, when the highly transcribed B-I allele is “paramutated”, or converted, to the silenced B′ state. siRNAs produced from the tandem repeats are hypothesized to mediate trans-interactions or communication between the alleles, as well as direct the establishment of a closed chromatin structure at the B-I tandem repeats. The conversion of B-I to a silenced epigenetic state is meiotically stable, and in the next generation all progeny will inherit a silenced B′ allele. The newly paramutated allele is termed B′*.