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Paramutation is the directed, heritable alteration of the expression of one allele when heterozygous with another allele. Here, the isolation and characterization of a mutation affecting paramutation, mediator of paramutation1-1 ( mop1-1 ), are described. Experiments demonstrate that the wild-type gene Mop1 is required for establishment and maintenance of the paramutant state. The mop1-1 mutation affects paramutation at the multiple loci tested but has no effect on alleles that do not participate in paramutation. The mutation does not alter the amounts of actin and ubiquitin transcripts, which suggests that the mop1 gene does not encode a global repressor. Maize plants homozygous for mop1-1 can have pleiotropic developmental defects, suggesting that mop1-1 may affect more genes than just the known paramutant ones. The mop1-1 mutation does not alter the extent of DNA methylation in rDNA and centromeric repeats. The observation that mop1 affects paramutation at multiple loci, despite major differences between these loci in their gene structure, correlations with DNA methylation, and stability of the paramutant state, suggests that a common mechanism underlies paramutation. A protein-based epigenetic model for paramutation is discussed. INTRODUCTIONRecognition of the widespread nature and importance of epigenetic phenomena in many organisms has prompted extensive review (see Cell Vol. 93, No. 2; Trends in Genetics Vol. 13, No. 8; Plant Molecular Biology Vol. 43, No. 2/3). Epigenetics refers to altered gene expression associated with alternative chromatin or methylation states (or both) superimposed on an unchanged primary DNA sequence. Alternative epigenetic states are frequently heritable through mitosis (Holliday et al., 1996;Pirrotta, 1997;Sherman and Pillus, 1997) and sometimes through meiosis (Jorgensen, 1995;Grewal and Klar, 1996;Cavalli and Paro, 1998;. Epigenetic modifications are often attributed to trans -sensing or homology effects (Henikoff and Comai, 1998;Wu and Morris, 1999). Some of the earliest examples of epigenetic phenomena were identified by pigment variegation in maize and Drosophila (Muller, 1930;Hinton and Goodsmith, 1950;Baker, 1953, Brink, 1956McClintock, 1957;Hessler, 1958;Coe, 1959).Anthocyanin pigmentation is a valuable tool in plants for identifying and tracking genetic and epigenetic events (Dooner et al., 1991;Meyer, 1995). The amount of the nonessential anthocyanins present is very sensitive to subtle changes in expression of the regulatory genes, and these differences are readily visible. In maize, these genes were used by Barbara McClintock to identify and follow the behavior of transposable elements (reviewed in Fedoroff, 1983) and by Alexander Brink and several researchers since to identify and characterize a phenomenon Brink (1958Brink ( , 1973 termed paramutation. More recently, the first examples of cosuppression-transgene silencing of a homologous endogenous gene-were identified by using anthocyanin genes in petunia (Napoli et al., 1990;van der Krol et al., 1990).Paramutat...

Paramutation is the directed, heritable alteration of the expression of one allele when heterozygous with another allele. Here, the isolation and characterization of a mutation affecting paramutation, mediator of paramutation1-1 (mop1-1), are described. Experiments demonstrate that the wild-type gene Mop1 is required for establishment and maintenance of the paramutant state. The mop1-1 mutation affects paramutation at the multiple loci tested but has no effect on alleles that do not participate in paramutation. The mutation does not alter the amounts of actin and ubiquitin transcripts, which suggests that the mop1 gene does not encode a global repressor. Maize plants homozygous for mop1-1 can have pleiotropic developmental defects, suggesting that mop1-1 may affect more genes than just the known paramutant ones. The mop1-1 mutation does not alter the extent of DNA methylation in rDNA and centromeric repeats. The observation that mop1 affects paramutation at multiple loci, despite major differences between these loci in their gene structure, correlations with DNA methylation, and stability of the paramutant state, suggests that a common mechanism underlies paramutation. A protein-based epigenetic model for paramutation is discussed.

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Sites of transposon Tn7 insertion in the Escherichia coli chromosome were examined, and two distinct classes of target sites differing in nucleotide sequence were identified. The target site choice was found to be determined by Tn7-encoded transposition genes.