7 Long-distance Cis and Trans interactions mediate paramutation

Abstract

This chapter focuses on the paramutation at two maize loci, b1 and p1. The chapter describes the recent experiments that have identified cis-acting regions required for paramutation at these two loci and compares the similarities and differences in the results. A model for the mechanism of paramutation is presented that incorporates these recent findings and discusses the possible functions for paramutation. Paramutation-like interactions can occur between two homologous transgenes, or a transgene and a homologous endogenous gene. At b1 , paramutation is fully penetrant and the resulting B ’ alleles are extremely stable, independent of how the alleles are maintained (homozygous or heterozygous with B-I or neutral alleles). Similar to the b1 paramutation, the paramutable P1-rr (red pericarp and red cob) allele can change spontaneously to a lower expression state, P1-pr (patterned pericarp and red cob). Models that attempt to explain paramutation must account for various features. Certain enhancer sequences are associated with the sequences required for paramutation in two different genes. The high-expressing alleles can be silenced spontaneously.