CHARACTERIZATION OF AN ARABIDOPSIS-THALIANA DNA HYPOMETHYLATION MUTANT

We have recently isolated two Arabidopsis thaliana DNA hypomethylation mutations, identifying the DDM1 locus, that cause a 70% reduction in genomic 5-methylcytosine levels [1]. Here we describe further phenotyp ic and biochemical characterization of the ddm1 mutants. ddm1/ddm1 hom ozygotes exhibited altered leaf shape, increased cauline leaf number, and a delay in the onset of flowering when compared to non-mutant sibl ings in a segregating population. Our biochemical characterization inv estigated two possible mechanisms for DNA hypomethylation. In order to see if ddm1 mutations affect DNA methyltransferase function, we compa red DNA methyltransferase activities in extracts from wild-type and dd m1 mutant tissues. The ddm1 mutant extracts had as much DNA methyltran sferase activity as that of the wild-type for both the Cpl and CpNpG s ubstrates suggesting that the DDM1 locus does not encode a DNA methylt ransferase. Moreover, the ddm1 mutations did not affect the intracellu lar level of S-adenosylmethionine, the methyl group donor for DNA meth ylation. The possibility that the DDM1 gene product functions as a mod ifier of DNA methylation is discussed.