Flowering represents a crucial transition from a vegetative to a reproductive phase of the plant life cycle. Despite extensive studies, the molecular mechanisms controlling flowering remain elusive. Although the enzymes involved are unknown, methylation of histone H3 K9 and K27 correlates with repression of FLOWERING LOCUS C (FLC), an essential transcriptional repressor involved in flowering time control in Arabidopsis thaliana; in contrast, methylation of H3K4 correlates with FLC activation. Here we show that loss-of-function of SET DOMAIN GROUP 8 (SDG 8), which encodes a homologue of the yeast SET2 histone methyltransferase, results in reduced dimethylation of histone H3K36, particularly in chromatin associated with the FLC promoter and the first intron, regions that contain essential cis-elements for transcription. sdg8 mutants display reduced FLC expression and flower early, establishing SDG8-mediated H3K36 methylation as a novel epigenetic memory code required for FLC expression in preventing early flowering. This is the first demonstrated role of H3K36 methylation in eukaryote development.
Partitioning and clustering are crucial steps in circuit layout for handling large scale designs enabled by the deep submicron technologies. Retiming is an important sequential logic optimization technique for reducing the clock period by optimally repositioning flipflops [7]. In our exploration of a logical and physical co-design flow, we developed a highly efficient algorithm on combining retiming with circuit partitioning or clustering for clock period minimization. Compared with the recent result by Pan et al. [lo] on quasioptimal clustering with retiming, our algorithm is able to reduce both runtime and memory requirement by one order of magnitude without losing quality. Our results show that our algorithm can be over 1000X faster for large designs.
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