Minisymposium 6: Epigenetics

Abs # 16003: Investigation of the role of plant PAF1-related proteins in transcription and histone methylation in plants

Presenter:	Oh, Sookyung , ohsookyu@msu.edu
Authors	Oh, Sookyung  (A) (B)  Ludwig, Philip  (A) (C)  Zhang, Hua  (A) (D)  van Nocker, Steven  (A) (E)  Yan, Ying  (A) (C)
Affiliations:	(A): Michigan State University (B): Program in Plant Breeding and Genetics (C): Program in Cell and Molecular Biology (D): Program in Genetics (E): Program in Plant Breeding and Genetics, Cell and Molecular Biology, and Genetics
Web Site:	http://vannocke.user.msu.edu/

In Arabidopsis thaliana, the promotion of flowering by extended exposure to cold temperatures, a phenomenon called vernalization, is mediated in part through silencing of the MADS-box transcriptional regulator gene FLC. Silencing of FLC in vernalized plants is maintained through an epigenetic mechanism involving specific ‘silencing’ histone modifications and a plant Polycomb-like mechanism. We formerly described a class of at least seven genes designated VERNALIZATION INDEPENDENCE (VIP) that when defective, confer vernalization-independent early flowering and constitutive silencing of FLC. In addition to the flowering-time defect, vip mutants exhibit developmental pleiotropy, suggesting that the respective VIP genes are important for normal expression of other regulatory genes as well. We carried out transcriptional profiling of vip mutants using the Affymetrix 'whole genome' genechip. We found that loss of function of VIP5 or VIP6 resulted in misregulation of a common and restricted subset of MADS-box genes. We cloned VIP5 and VIP6, and found that the encoded proteins are closely related to Rtf1 and Ctr9 from budding yeast. These proteins are components of the transcriptional regulator PAF1, which plays a key role in transcription by mediating the establishment and/or maintenance of specific chromatin modifications associated with gene activation. For example, components of PAF1 are involved in ubiquitylation of histone H2B by RAD6-BRE1, and methylation of the N-terminal tail of histone H3 at lysine 4 (H3K-4) by the Trithorax-like histone methyltransferase Set1 and lysine 79 (H3K-79) by Dot1. In fruit flies, where best characterized, Trithorax group proteins act antagonistically to Polycomb group proteins to maintain the epigenetic activity of Hox developmental regulatory genes. We hypothesize that VIP5/VIP6 and other VIP proteins may assist in maintaining FLC activity by participating in a plant Trithorax-associated mechanism. We investigated conservation of VIP5/VIP6 function through analysis of the effect of the vip5/vip6 mutations on H3K-4 or H3K-79 methylation both at the 'global' level, and within potential VIP5/VIP6 regulatory targets, including FLC. Our observations suggest that opposing Polycomb-and Trithorax-related mechanisms of gene regulation may converge on MADS-box genes in plants.