Covalent modifications of histone tails modulate gene expression via chromatin organization. As examples, methylation of lysine 9 residues of histone H3 (H3) (H3-K9) is believed to repress transcription by compacting chromatin, whereas methylation of lysine 4 residues of H3 (H3-K4) is believed to activate transcription by relaxing chromatin. The Drosophila trithorax group protein absent, small, or homeotic discs 1 (ASH1) is involved in maintaining active transcription of many genes. Here we report that in extreme ash1 mutants, no H3-K4 methylation is detectable. Within the limits of our assays, this lack of detectable H3-K4 methylation implies that ASH1 is required for essentially all H3-K4 methylation that occurs in vivo. We report further that the 149-aa SET domain of ASH1 is sufficient for H3-K4 methylation in vitro. These findings support a model in which ASH1 is directly involved in maintaining active transcription by conferring a relaxed chromatin structure. C hromatin serves as the template for processing genetic information. The dynamic DNA-protein structure of chromatin is influenced by epigenetic modifications on both the DNA and nucleosomal histones. Chromatin can exist in alternate states: a relaxed state (euchromatin) where underlying DNA becomes accessible to transcription factors, and a condensed state (heterochromatin) where underlying DNA is transcriptionally restricted. The functional organization of chromatin structure is in part the result of targeted covalent modifications on the N-terminal tails of the core histones. The histone code hypothesis suggests that combinations of histone modifications format the chromatin for proper gene regulation (1).Although histone methylation was first described in 1964 (2), its significance for regulating chromatin structure and transcription remained unclear until the discovery of the first histone methyltransferase (HMTase), SUV39H (3), a SET domaincontaining protein. Lysine-directed histone methylation is intriguing, because it is involved in mediating both compacted and relaxed chromatin. Histone H3 (H3) can be methylated at lysine residues 4, 9, 27, and 36. Methylation of lysine 9 modulates gene silencing (3), whereas methylation of lysine 4 appears to function as a signal for gene activation (4). It has been proposed that the combination of methylation on the lysine 4 and 9 residues of histone H3 with methylation on the lysine 20 residue of histone H4 may also serve as a signal for gene activation (5).The SET domain is present in transcriptional regulators that have opposing functions. For example, consider the three Drosophila proteins that originally defined the SET domain: suppressor of variegation 3-9 [SU(VAR)3-9] is involved in heterochromatin-dependent gene silencing (6, 7), enhancer of zeste, is involved in maintaining both activated and repressed transcriptional states (8), and trithorax (TRX) is involved in maintaining gene activation (9). The HMTase function of the SET domain has been implicated in conferring both active and repressed transc...

An effort has been made to study the lipid transformations, especially acyl groups, during two important changes in leaves. These two changes were greening and senescence. Since no one single tissue, in general, lends itself to such a study, two kinds of leaf tissue were selected-barley (Hordeum vulgare L.) and Cocklebur (Xanthium strumarium L.). These two changes are of interest since one is a phase in which photosynthesis is initiated and the other is a phase during which photosynthesis shuts down. To follow changes in the fatty acid composition during greening, barley leaf tissue was used. An effort was made to alter the lag phase in the chlorophyll and fatty acid changes by treating the tissue with red, far red, or red followed by far red light prior to a continuous white light treatment. As opposed to following the greening process, several experiments were conducted to follow lipid changes in the senescent phase of photosynthetic tissues. For example, the changes in pigments and fatty acids of green barley leaf tissue placed in the dark were followed. Placing the barley leaf tissue in the dark causes a simulated senescence effect. To continue the study of senescence, cocklebur leaf discs were floated on an aqueous medium. Cocklebur is an excellent choice since the excised leaf discs senesce rapidly without a concomitant necrosis. We also followed the rate of acetate-`C incorporation into senescing cocklebur leaf lipids such as MGDG2, DGDG, and the phospholipids.Unser and Mohr (11) ated an increase in MGDG and DGDG in mustard seedlings. We wondered if a red light treatment might reduce the lag effect in chlorophyll synthesis as well as in the fatty acid changes which are associated with greening. We have found that if greening is prevented red light did not reduce the lag period for the synthesis of linolenate (8). However, no attempt was made to allow chlorophyll synthesis to occur while determining the effect of a prior red light treatment on the subsequent fatty acid synthesis. Akoyunoglou (1) found that there was a pronounced effect of red and far red light treatment on chlorophyll synthesis in old leaves; no effect was observed in young leaves. We therefore allowed the plants to green while studying the effect of a prior red light treatment on the fatty acid changes. Concerning the senescent phase, Draper (2) found that the final stages of this phase in cucumber cotyledons are marked by a loss of all classes of lipids. We have attempted to make a rough estimate of acetate incorporation and carbon retention in the lipids in senescing cocklebur leaf discs treated with either sucrose or abscisic acid. Khudairi (4) found that leaf discs floated on sucrose showed a more rapid loss of chlorophyll than those floated on water. We tried the same experiment using RNase-free sucrose. We wondered if the sucrose would also hasten changes in the lipids which were characteristic of the senescence phase. The abscisic acid was used to accelerate the senescence process. MATERIALS AND METHODSGrowth of Plants and Light T...