Symposium I: New Advances & Insights in Plant Biology

Abs # 10003: Imprinting: at the intersection of DNA repair and gene transcription

Presenter:	Fischer, Robert L, rfischer@uclink.berkeley.edu
Authors	Fischer, Robert L (A)   Choi, Yeonhee  (A)   Gehring, Mary  (A)
Affiliations:	(A): Department of Plant and Microbial Biology, University of California at Berkeley
Web Site:	http://plantbio.berkeley.edu/

Genomic imprinting causes genes to be expressed according to their parental origin and is observed exclusively in mammals and flowering plants. In mammals, many of the imprinted genes are expressed in the extraembryonic membranes that transfer nutrients from the mother to the embryo. In plants, the endosperm performs a similar function and is the critical site of gene imprinting. In mammals the mechanism of gene imprinting involves the differential methylation (5-methylcytosine) of alleles. In plants the mechanisms that control imprinting are unknown but both DNA methylation-dependent and -independent mechanisms regulate epigenetic phenomena. We have discovered that the DEMETER (DME) gene mediates endosperm imprinting in Arabidopsis (Choi et al., Cell (2002) 110:33-42). Seed viability depends only on the maternal DME allele. DME encodes a large protein with DNA glycosylase and nuclear localization domains. Most DNA glycosylases function in DNA repair and excise modified, damaged, or mispaired bases, creating an abasic site. AP endonuclease and DNA polymerase complete the repair process by nicking the DNA and by replacing the abasic site with a new base, respectively. We find that the DME DNA glycosylase functions in vivo to activate maternal allele transcription of imprinted genes. DME transcription is restricted to the central cell of the female gametophyte, the progenitor of the endosperm. DME is required for maternal allele expression of the imprinted MEDEA (MEA) Polycomb gene in the central cell and endosperm. Ectopic DME expression induces MEA transcription and analysis of the MEA promoter reveals DME-induced nicking at multiple sites. Mutagenesis of the DME DNA glycosylase active site verifies that base excision activity is essential for DME function in vivo. Thus, the DME DNA glycosylase activates maternal expression of imprinted genes in the central cell, a process that is essential for seed viability.