In eukaryotic organisms, processes of development and differentiation are epigentically controlled at the level of higher order chromatin structure. Changes in packaging of chromatin domains lead to activation or repression of single genes within such domains. Numerous studies have shown that several phenomena of homology dependent transcriptional gene silencing in transgenic plants are also the consequence of chromatin remodelling. Isolation of mutants affecting such silencing processes should lead to the identification of proteins involved in establishment and regulation of epigenetic structures. According to this hypothesis, the aim of this work was the isolation and characterisation of new suppressor mutants for transcriptional gene silencing (TGS). Due to the lack of sensitive and facile test systems for TGS in plants, a new silencing system based on transcriptionaly silenced transgenic reporter genes (GUS, Luciferase) was established. For this purpose, Arabidopsis-T-DNA lines have been constructed containing one to four copies of the same reporter gene driven by the CaMV 35S promotor. To isolate suppressor mutants for TGS, EMS mutagenesis using completly inactivated luciferase lines has been carried out. 50,000 M2 plants raised from 50 independent pools were screened for increased luciferase activity and 159 putative mutants were isolated. In 9 of 19 further characterised mutants, the DNA methylation of the luciferase genes is largely reduced compared to non-mutagenised control plants. Mapping experiments on 15 mutant lines allow to limit 8 different loci as putative mutation sites. The mapping data of 9 lines with characteristic changes in DNA methylation suggest that these mutants are allelic to the known silencing mutants ddm1, met1 and cmt3. The mutation sites of three other mutants without changes in DNA methylation were discovered by map based cloning. Two of these mutants showing tissue specific TGS suppression were point mutations in TTG2. Bru1-2, the third new mutant, was characterised in collaboration with other groups.