mediator of paramutation1 is required for establishment and maintenance ofparamutation at multiple maize loci

Paramutation is the directed, heritable alteration of the expression of one allele when heterozygous with another allele. Here, the isolation and char acterization of a mutation affecting paramutation, mediator of paramutation 1-1 (mop1-1), are described. Experiments demonstrate that the wild-type gen e Mop1 is required for establishment and maintenance of the paramutant stat e. The mop1-1 mutation affects paramutation at the multiple loci tested but has no effect on alleles that do not participate in paramutation. The muta tion does not alter the amounts of actin and ubiquitin transcripts, which s uggests that the mop1 gene does not encode a global repressor. Maize plants homozygous for mop1-1 can have pleiotropic developmental defects, suggesti ng that mop1-1 may affect more genes than just the known paramutant ones. T he mop1-1 mutation does not alter the extent of DNA methylation in rDNA and centromeric repeats. The observation that mop1 affects paramutation at mul tiple loci, despite major differences between these loci in their gene stru cture, correlations with DNA methylation, and stability of the paramutant s tate, suggests that a common mechanism underlies paramutation. A protein-ba sed epigenetic model for paramutation is discussed.