Epigenetische Mechanismen zur Regulation der Transkription C 4-typischer Gene in Mais (Zea mays L.)
Nuclear genes are organised in chromatin. The transcriptional activation of chromatin is associated with various epigenetic mechanisms and covalent histone modifications play a central role in this process. Among these modifications, methylation and acetylation of the N-terminal regions of histones H3 and H4 are most important. Acetylation of histones normally correlates with transcriptional activation whereas histone methylation can show either activating or repressing effects. Methylation of DNA can also act as a signal that is often associated with gene repression. An appropriate model system for the analysis of epigenetic effects on transcription are genes involved in the C4-metabolism. As these genes show a light-induced and tissue-specific gene regulation, different transcriptional levels can be easily induced and analysed. In this study, the transcriptional activation of C4-PEPC, C4-ME, C4-PPDK and the small subunit of RubisCO RbcS2 was determined by Real-Time-PCR. Histone methylation was analysed via ChIP-analysis and DNA methylation patterns were measured with restriction enzymes sensitive or insensitive to cytosine methylation. Upon light induction, all genes studied are expressed either in the mesophyll or in bundle sheath cells. An impact of DNA methylation on light-dependent or tissue-specific regulation of transcription could not be detected, but a strong correlation of certain histone modifications with the potential to activate genes in specific tissues was observed. Both coding and promotor regions of the tested genes showed trimethylation of lysine 4 on histone H3 (=H3K4) in the tissues where the genes could be activated by light. In contrast, tissues where transcription could not be activated display increased values of mono- and dimethylation of the lysine residue. These modifications were independent of transcription itself, in particular light was not necessary to establish the modification pattern. The strongest effects were monitored at the genes encoding C4-PEPC and C4-ME, but the same tendencies were observed at the genes for C4-PPDK and RbcS2. Unexpectedly, dimethylation of lysine 9 on histone H3, a modification normally associated with repressed chromatin, was also found on the C4-PEPC-gene. Signals were as strong as measured at a transcriptionally totally inactive retrotransposon sequence. Comparison of the histone methylation patterns in leaf and root tissues indicates a strong reduction of H3K4-trimethylation in the root. Final results could not be obtained for the other modifications. Data are discussed against the background of the histone code hypothesis and a model is suggested where different environmental stimuli are integrated into a promoter response through the induction of specific histone modifications.
info:eu-repo/classification/ddc/570, Allgemeine Botanik, Genetik, Histon-Methyltransferase, Histon-Acetyltransferase, Histon-DNS-Komplex, DNS-Methyltransferase, DNS-Bindung, DNS, Biowissenschaften, Biologie, DNA-Methylierung, Chromatin-Immunopräzipitation, Histon H3, Mais, Lysin, DNA methylation, chromatin immunoprecipitation, histone H3, maize, lysine
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