Untersuchung des Zusammenhanges zwischen der Funktion pflanzlicher microRNAs und der DNA-Methylierung des PHABULOSA Locus in Arabidopsis thaliana

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This proposal aims to unravel the biological function of genie methylation observed at the PHABULOSA (PHB) locus in Arabidopsis thaliana. PHB is a key transcription factor required for the establishment of organ polarity in the Arabidopsis leaf. Recently it has been shown that the levels of PHB messenger RNA (mRNA) are controlled by microRNA165/166 (McConnell et al., 2001; Rhoades et al.f 2002; Mallory et al., 2004) and that microRNA/mRNA-interaction is required for methylation of a coding region within the PHB locus downstream of the microRNA complementary site (Bao et al., 2004). Although siRNAs are known to direct methylation of homologous DMA sequences (Mette et al., 2000), methylation of DMA is a function which has so far not been ascribed to microRNAs. Furthermore the machinery involved in the microRNA dependent methylation process appears to be different from that responsible for siRNA-mediated methylation as the sequences are not homologous to the miRNA and are more distantly located downstream. This points towards a novel feature of microRNAs being able to direct chromosomal modification in cis. We do not currently understand the biological role of this coding sequence methylation at the PHB locus. However, lower downstream methylation levels were observed in DNA samples made from tissues enriched for stem cells indicating that the methylation may be a developmentally controlled process. Our aim is to unravel the biological role and identify proteins involved in the methylation process hoping to gain a deeper understanding of how microRNAs control important developmental processes.