Research Projects with Alfalfa and other plants
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Slides
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Histone Synthesis and Turnover in Alfalfa. Fast Loss of Highly Acetylated Replacement Histone Variant H3.2
Waterborg, JH
J. Biol. Chem. 1993 Mar 5; 268(7): 4912-4917
Histone synthesis in alfalfa tissue culture cells was studied by labeling with tritiated lysine, purification of histone proteins by reversed-phase high pressure liquid chromatography, and fluorography of acid/urea/Triton X-100 polyacrylamide gels. Minor histone variant H3.2 was synthesized twice as fast as major variant H3.1. The predicted difference in histone H3 variant turnover was examined during continued growth. More than 50% of newly synthesized histone H3.2 and 20% of new H3.1 were lost from chromatin over a period of 100 h. This produced a ratio between the stable remaining portions of each new histone H3 variant protein identical to that of the steady-state histone H3 variants. The labile portion of new histone H3.2 (half-life of 20 h) was rapidly lost specifically from transcriptionally active chromatin as judged by the acetylation level of nearly 1.5 acetylated lysines/histone molecule, a level 50% higher than the acetylation in histone H3.2 overall and three times that of histone H3.1. These results and the constitutive level of H3.2 gene expression identify histone H3.2 of alfalfa as a functional replacement histone variant. The extent of its preferential assembly into active chromatin nucleosomes and the rapid rate of its subsequent loss indicate significant dissolution of plant nucleosomes during gene transcription.
Dynamic Methylation of Alfalfa Histone H3
Waterborg, JH
J. Biol. Chem. 1993 Mar 5; 268(7): 4918-4821
Dynamic lysine methylation of histone H3 in alfalfa tissue culture cells was studied by labeling with tritiated methionine, purification of variants H3.1 and H3.2 by reversed-phase high pressure liquid chromatography and amino acid analysis. Mono- and dimethyl-L-lysine were the major labeled amino acids. Within 100 h of continued growth conversion from N epsilon-monomethyl-L-lysine (MML) to N epsilon-dimethyl-L-lysine (DML) and N epsilon-trimethyl-L-lysine (TML) was observed, consistent with steady-state histone methylation. During the same time 20% of the methylation label was lost from major variant H3.1 protein and more than 50% from the more highly labeled minor variant H3.2. A similar pattern of label incorporation and loss was observed during a study of histone synthesis and turnover. This conforms with the general observation in animal cells that lysine methylation is limited to newly synthesized histone. Increased methylation of the more highly acetylated forms of histone H3 protein indicates limited accessibility of chromatin for histone methylation. After loss of the labile fraction of newly synthesized H3 variants, stably methylated proteins with 30% of the label in MML, 40% in DML, and 25% in TML remain. Turnover of methyl modification groups independent of histone turnover was not detected.
Sequence Analysis of Acetylation and Methylation in Two Histone H3 Variants of Alfalfa
Waterborg, JH
J. Biol. Chem. 1990 Oct 5; 265(28): 17157-17161
Analysis of acetylation in the two histone H3 variants of alfalfa by acid/urea/Triton-polyacrylamide gel electrophoresis has established that the minor variant H3.2 has a 2-fold higher level of acetylation than the major variant H3.1. Purification and sequence analysis of both variants showed sequence identity across the complete amino-terminal domain, which contains the 6 modified lysines 4, 8, 14, 18, 23, and 27. The two proteins have different distributions for acetylation: mono-, di-, and tri-methylation. The higher level of acetylation of H3.2 was confirmed in a wider pattern across all 6 lysines. Lysine modification levels varied for all sites in both proteins between 5 and 95%, with combinations of one to four types of modification co-existing at each residue. Additional sequence analysis of the H3.1 and H3.2 proteins and of tryptic core peptides established that the two histones differ only in residues 31, 41, 87, and 90. This indicates that major histone H3.1 is the product of the major alfalfa histone H3 gene and makes it likely that H3.2 is the product of the minor H3 gene, known from a partial cDNA clone. The variant-specific differences in lysine modifications in protein domains with identical primary structures suggest that the pattern and level of lysine modifications may be directed by the distinct chromatin environments of the two histone H3 variants.
Existence of Two Histone H3 Variants in Dicotyledonous Plants and Correlation between Their Acetylation and Plant Genome Size
Waterborg, JH
Plant Mol. Biol. 1992 Jan; 18(2): 181-187
Histone H3 proteins were purified to near homogeneity from callus cultures of dicotyledonous plants alfalfa, soybean, Arabidopsis, carrot and tobacco to determine the number of histone H3 variants. In every species two histone H3 variants were identified by gradient gel electrophoresis and reversed-phase chromatography. They were named H3.1 and H3.2 in order of increasing mobility in acid-urea-Triton gels. Co-electrophoresis of histone H3.2 proteins of all species in this gel system and HPLC co-chromatography suggest that all histone H3.2 variants have a primary protein sequence identical to alfalfa H3.2. Two distinct H3.1 variant forms were identified, represented by alfalfa and Arabidopsis H3.1 proteins which differ only at residue 90. Soybean H3.1 resembles H3.1 of alfalfa. Carrot and tobacco H3.1 appear identical to the Arabidopsis H3.1 histone variant. All H3 proteins were acetylated to multiple levels and in each plant the histone H3.2 forms were more highly acetylated. An inverse relationship was observed between plant genome size and the relative abundance of histone variant H3.2 and also with the level of acetylation of both histone H3 variants. This correlation matches the general tendency that in plants with smaller genomes a larger fraction of the genome is transcriptionally active.
Identification of Five Sites of Acetylation in Alfalfa Histone H4
Waterborg, JH
Biochemistry 1992 Jul 14; 31(27): 6211-6219
Radioactive acetylation in vivo of plant histone H4 of alfalfa, Arabidopsis, tobacco, and carrot revealed five distinct forms of radioactive, acetylated histone. In histone H4 of eukaryotes ranging from fungi to man, acetylation is restricted to four lysines (residues 5, 8, 12, and 16) possibly caused by a quantitative methylation of lysine-20. Chemical and proteolytic fragmentation of the amino terminally blocked alfalfa H4 protein, dynamically acetylated by radioactive acetate in vivo, allowed protein sequencing and identification of selected peptides. Peptide identification was facilitated by analyzing fully characterized calf histone H4 in parallel. Acetylation in vivo of alfalfa histone H4 was restricted to the lysines in the amino-terminal domain of the protein, residues 1-23. Lysine-20 was shown to be free of methylation, as in pea histone H4. This apparently makes lysine-20 accessible as a novel target for histone acetylation. The in vivo pattern of lysine acetylation (16 > 12 > 8 >= 5 = 20) revealed a preference for lysines -16 and -12 without an apparent strict sequential specificity of acetylation.
Common Features of Analogous Replacement Histone H3 Genes in Animals and Plants
Waterborg, JH; Robertson, AJ
J. Mol. Evol. 1996 Sep; 43(3): 194-206
Phylogenetic analysis of histone H3 protein sequences demonstrates the independent origin of the replacement histone H3 genes in animals and in plants. Multiple introns in the replacement histone H3 genes of animals in a pattern distinct from that in plant replacement H3 genes supports this conclusion. It is suggested that replacement H3 genes arose at the same time that, independently, multicellular forms of animals and of plants evolved. Judged by the degree of invariant and functionally constrained amino acid positions, histones H3 and H4, which form together the tetramer kernel of the nucleosome, have co-evolved with equal rates of sequence divergence. Residues 31 and 87 in histone H3 are the only residues that consistently changed across each gene duplication event that created functional replacement histone H3 variant forms. Once changed, these residues have remained invariant across divergent speciation. This suggests that they are required to allow replacement histone H3 to participate in the assembly of nucleosomes in non-S-phase cells. The abundant occurrence of polypyrimidine sequences in the introns of all replacement H3 genes, and the replacement of an intron by a polypyrimidine motif upstream of the alfalfa replacement H3 gene, suggests a function. It is speculated that they may contribute to the characteristic cell-cycle-independent pattern of replacement histone H3 genes by binding nucleosome-excluding proteins.
Identification of Three Highly Expressed Replacement Histone H3 Genes of Alfalfa
Robertson, AJ; Kapros, T; Dudits, D; Waterborg, JH
DNA Seq. 1996; 6(3): 137-146
One genomic and six cDNA clones for the replacement histone H3.2 protein of alfalfa (Medicago sativa) were isolated and sequenced. By gene organization they represent 3 distinct genes. PCR methods were used to confirm that only three intron-bearing histone H3.2 genes of this type exist per haploid genome. They co-exist with approximately 56 copies of the previously characterized replication-dependent, intronless histone H3.1 variant gene. Comparison of the relative expression of few constitutive H3.2 genes with the high S phase expression of the abundant cell cycle-dependent H3.1 genes by mRNA levels and protein synthesis measurements revealed that the replacement histone H3.2 genes are very highly expressed. Structural analysis of the genomic replacement H3.2 gene revealed a unique feature. A repeated polypyrimidine sequence motif in the 5' untranslated region of this gene replaces the ubiquitous intron present in all known replacement H3 genes. A hypothesis is presented that this motif and other, non-randomly distributed polypyrimidine sequences in the introns of replacement histone H3 genes of alfalfa and Arabidopsis, may affect nucleosome assembly. Chromatin repression of these replacement genes would be avoided, consistent with the high, constitutive expression of replacement H3 histone genes in plants.
A Cell Cycle-regulated Histone H3 Gene of Alfalfa with an Atypical Promoter Structure
Robertson, AJ; Kapros, T; Waterborg, JH
DNA Seq. 1997; 7(3-4): 209-216
The control of cell cycle expression of histone genes in plants is incompletely understood. A new histone H3 gene was cloned from alfalfa (Medicago sativa) that codes for the replication-dependent histone H3.1 variant protein. Despite lacking all promoter sequence motifs that have been associated with cell cycle-dependent histone gene expression in plants, northern analysis of synchronized cells clearly linked gene expression to DNA replication. TTAATNA was recognized as a new sequence element in the 3' untranslated regions of this and all other cell cycle-dependent histone H3 genes of dicotyledonous plants. It is not found in the replication-independent histone H3 genes.
Histone H3 Transcript Stability in Alfalfa
Kapros, T; Robertson, AJ; Waterborg, JH
Plant Mol. Biol. 1995 Aug; 28(5): 901-914
The stability of histone H3 transcripts in alfalfa for replication-dependent and -independent gene variants was measured by northern analysis under conditions of inhibition of transcription and/or translation. Replication-dependent histone H3.1 transcripts were about three-fold less stable than the equally polyadenylated mRNA for replacement variant H3.2 histone. In actively growing suspension cultures treated with dactinomycin half-lives of 2 and 7 h were observed for H3.1 and H3.2 mRNAs, respectively. mRNA stabilities were also measured indirectly by histone protein synthesis. The translation inhibitor cycloheximide strongly increased mRNA levels for both histone H3 variants. The dependence of histone mRNA turnover on translation in animals also appears to exist in plants. The combination of inhibition of transcription and translation by dactinomycin and cycloheximide was used in an indirect assessment of H3 mRNA stability throughout the cell cycle in partially synchronized and cycle-arrested cultures. Destabilization of replication-dependent histone H3.1 mRNA was detected in non-S phase cells.
Dynamic Histone Acetylation in Alfalfa Cells. Butyrate Interference with Acetate Labeling
Waterborg, JH; Harrington, RE; Winicov, I
Biochim. Biophys. Acta 1990 Jul 30; 1049(3): 324-330
Dynamic histone acetylation of alfalfa (Medicago sativa) was studied in suspension cultures by short-term labeling with radioactive acetate. The relative labeling rates for the acetylated histones were in order of decreasing incorporation; H3.2 greater than H3.1 greater than H4 greater than H2B.1 greater than H2A.3. Histone H3 showed at least seven sites of acetylation, histone H2B.1 had six sites and histone H4 had five sites. Low numbers of acetylation sites were observed for histone H2B.2 and all histone H2A variants. The mass ratio, steady state acetylation and dynamic acetylation between major variant H3.1 and minor variant H3.2 were approx. 2:1, 1:2 and 2:5, respectively. Treatment of alfalfa cells with 50 mM n-butyrate did not lead to histone hyperacetylation, but instead interfered with histone acetylation labeling by acetate. The extent of apparent inhibition increased with time and concentration of butyrate. It is likely that the conversion of butyrate to acetylCoA results in dilution of the specific radioactivity of [3H]acetate in the acetylCoA pool thereby inhibiting the labeling reaction. This interpretation is supported by 14C-labeling of alfalfa acetylated histones by [1-14C]butyrate.