Paramutation occurs at a specific locus when one epiallele, referred to as paramutagenic, facilitates a heritable change in the regulation of the other epiallele, which is referred to as paramutable (reviewed in Arteaga-Vazquez and Chandler, 2010). Because these two states have the same DNA sequence but differ epigenetically, they are referred to as epialleles, or states, rather than alleles. Generally, transcription from paramutable epialleles is heritably repressed following exposure to paramutagenic alleles. Paramutation was first discovered in the 1950s as an epigenetic phenomenon affecting maize (Zea mays) anthocyanin production regulated by red1 (r1) and booster1 (b1). For example, the paramutable B-I epiallele produces strong anthocyanin expression, but when crossed to the low-anthocyanin paramutagenic B' epiallele, the B-I allele is converted in trans into a heritable, low-anthocyanin, paramutagenic B' epiallele. Paramutation seems to involve sequences affecting transcription, such as an 853-bp tandem repeat 100 kb upstream of B-I, which acts as a transcriptional enhancer.
展开▼
