Evidence for an intermediate methyl-acceptor for chemotaxis in Bacillus subtilis

Taxis in archaea

Microorganisms can move towards favorable growth conditions as a response to environmental stimuli. This process requires a motility structure and a system to direct the movement. For swimming motility, archaea employ a rotating filament, the archaellum. This archaea-specific structure is functionally equivalent, but structurally different, from the bacterial flagellum. To control the directionality of movement, some archaea make use of the chemotaxis system, which is used for the same purpose by bacteria. Over the past decades, chemotaxis has been studied in detail in several model bacteria. In contrast, archaeal chemotaxis is much less explored and largely restricted to analyses in halophilic archaea. In this review, we summarize the available information on archaeal taxis. We conclude that archaeal chemotaxis proteins function similarly as their bacterial counterparts. However, because the motility structures are fundamentally different, an archaea-specific docking mechanism is required, for which initial experimental data have only recently been obtained.

Receptor conformational changes enhance methylesterase activity during chemotaxis by Bacillus subtilis

Addition and removal of the attractant asparagine causes methanol formation as a consequence of methylation and demethylation of conserved glutamate residues in the Bacillus subtilis chemotaxis receptor McpB C-terminal domain. We found that methanol was released on both addition and removal of asparagine even when the response regulator domain of CheB was removed (to produce CheB(141-357)). Thus, in undergoing the transition from unbound receptor to ligand-bound adapted receptor, the receptor must pass through a state of heightened susceptibility to demethylation by CheB that is independent of phosphorylation. The same result occurred when the aspartate phosphorylation site of CheB, Asp54, had been mutated to an asparagine residue, provided the enzyme was sufficiently induced. However, no methanol release was observed for an active site point mutant, cheB(S173C), in response to addition or removal of asparagine even when induced. Finally, methanol release was observed only for attractant addition in a mutant background lacking the coupling proteins, CheW and CheV, provided CheB(141-357) was present. Thus, on attractant addition, methanol must arise from a transient conformation of the receptor C-terminal domain that is an intrinsic property of the receptor; on attractant removal, however, methanol must arise from a different transient conformation, one dependent on the presence of coupling proteins.

The role of heterologous receptors in McpB-mediated signalling in Bacillus subtilis chemotaxis

Asparagine chemotaxis in Bacillus subtilis appears to involve two partially redundant adaptation mechanisms: a receptor methylation-independent process that operates at low attractant concentrations and a receptor methylation-dependent process that is required for optimal responses to high concentrations. In order to elucidate these processes, chemotactic responses were assessed for strains expressing methylation-defective mutations in the asparagine receptor, McpB, in which all 10 putative receptors (10del), five receptors (5del) or only the native copy of mcpB were deleted. This was done in both the presence and the absence of the methylesterase CheB. We found that: (i) only responses to high concentrations of asparagine were impaired; (ii) the presence of all heterologous receptors fully compensated for this defect, whereas responses progressively worsened as more receptors were taken away; (iii) methyl-group turnover occurred on heterologous receptors after the addition of asparagine, and these methylation changes were required for the restoration of normal swimming behaviour; (iv) in the absence of the methyleste-rase, the presence of heterologous receptors in some cases caused impaired chemotaxis; and (v) either a certain threshold number of receptors must be present to promote basal CheA activity, or one or more of the receptors missing in the 10del background (but present in the 5del background) is required for establishing basal CheA activity. Taken together, these findings suggest that many or all chemoreceptors work as an ensemble that constitutes a robust chemotaxis system. We propose that the ability of non-McpB receptors to compensate for the methylation-defective McpB mutations involves lateral transmission of the adapted conformational change across the ensemble.

Selective methylation changes on the Bacillus subtilis chemotaxis receptor McpB promote adaptation

The Bacillus subtilis McpB is a class III chemotaxis receptor, from which methanol is released in response to all stimuli. McpB has four putative methylation sites based upon the Escherichia coli consensus sequence. To explore the nature of methanol release from a class III receptor, all combinations of putative methylation sites Gln(371), Gln(595), Glu(630), and Glu(637) were substituted with aspartate, a conservative substitution that effectively eliminates methylation. McpB((Q371D,E630D,E637D)) in a Delta(mcpA mcpB tlpA tlpB)101::cat mcpC4::erm background failed to release methanol in response to either the addition or removal of the McpB-mediated attractant asparagine. In the same background, McpB((E630D,E637D)) produced methanol only upon asparagine addition, whereas McpB((Q371D,E630D)) produced methanol only upon asparagine removal. Thus methanol release from McpB was selective. Mutants unable to methylate site 637 but able to methylate site 630 had high prestimulus biases and were incapable of adapting to asparagine addition. Mutants unable to methylate site 630 but able to methylate site 637 had low prestimulus biases and were impaired in adaptation to asparagine removal. We propose that selective methylation of these two sites represents a method of adaptation novel from E. coli and present a model in which a charged residue rests between them. The placement of this charge would allow for opposing electrostatic effects (and hence opposing receptor conformational changes). We propose that CheC, a protein not found in enteric systems, has a role in regulating this selective methylation.

CheB is required for behavioural responses to negative stimuli during chemotaxis in Bacillus subtilis

The methyl-accepting chemotaxis protein, McpB, is the sole receptor mediating asparagine chemotaxis in Bacillus subtilis. In this study, we show that wild-type B. subtilis cells contain approximately 2,000 copies of McpB per cell, that these receptors are localized polarly, and that titration of only a few receptors is sufficient to generate a detectable behavioural response. In contrast to the wild type, a cheB mutant was incapable of tumbling in response to decreasing concentrations of asparagine, but the cheB mutant was able to accumulate to low concentrations of asparagine in the capillary assay, as observed previously in response to azetidine-2-carboxylate. Furthermore, net demethylation of McpB is logarithmically dependent on asparagine concentration, with half-maximal demethylation of McpB occurring when only 3% of the receptors are titrated. Because the corresponding methanol production is exponentially dependent on attractant concentration, net methylation changes and increased turnover of methyl groups must occur on McpB at high concentrations of asparagine. Together, the data support the hypothesis that methylation changes occur on asparagine-bound McpB to enhance the dynamic range of the receptor complex and to enable the cell to respond to a negative stimulus, such as removal of asparagine.

CheY-dependent methylation of the asparagine receptor, McpB, during chemotaxis in Bacillus subtilis

For the Gram-positive organism Bacillus subtilis, chemotaxis to the attractant asparagine is mediated by the chemoreceptor McpB. In this study, we show that rapid net demethylation of B. subtilis McpB results in the immediate production of methanol, presumably due to the action of CheB. We also show that net demethylation of McpB occurs upon both addition and removal of asparagine. After each demethylation event, McpB is remethylated to nearly prestimulus levels. Both remethylation events are attributable to CheR using S-adenosylmethionine as a substrate. Therefore, no methyl transfer to an intermediate carrier need be postulated to occur during chemotaxis in B. subtilis as was previously suggested. Furthermore, we show that the remethylation of asparagine-bound McpB requires the response regulator, CheY-P, suggesting that CheY-P acts in a feedback mechanism to facilitate adaptation to positive stimuli during chemotaxis in B. subtilis. This hypothesis is supported by two observations: a cheRBCD mutant is capable of transient excitation and subsequent oscillations that bring the flagellar rotational bias below the prestimulus value in the tethered cell assay, and the cheRBCD mutant is capable of swarming in a Tryptone swarm plate.

Primary structure and functional analysis of the soluble transducer protein HtrXI in the archaeon Halobacterium salinarium

Signal transduction in the archaeon Halobacterium salinarium is mediated by a family of 13 soluble and membrane-bound transducers. Here, we report the primary structure and functional analysis of one of the smallest halobacterial putative transducers, HtrXI. Hydropathy plot analysis of the primary structure predicts no membrane-spanning segments in HtrXI. The fractionation of the H. salinarium proteins confirmed that HtrXI is a soluble protein. Capillary assay with an HtrXI deletion mutant and a complemented strain revealed that this soluble transducer is involved in Asp and Glu taxis. In vivo analysis of the methylesterase activity of the htrXI-1 deletion mutant suggests that HtrXI plays an important role in the adaptation of the chemotactic responses to His, Asp, and Glu, which are attractants for halobacteria. Stimulation by Asp and Glu causes demethylation of HtrXI and of another putative transducer, HtrVII. But addition of His to halobacterial cells increases HtrXI methylation together with that of other putative transducers. In the absence of HtrXI, stimulation by either Glu or His does not decrease or increase the methylation of any putative transducers. Therefire, the HtrXI transducer appears to have a complex role in chemotaxis signal transduction.

A methyl-accepting protein involved in multiple-sugar chemotaxis by Cellulomonas gelida

Tethered-cell and capillary assays indicated that L-methionine is required by Cellulomonas gelida for its normal cell motility pattern and chemotaxis and that S-adenosylmethionine is involved in sugar chemotaxis by this cellulolytic bacterium. In addition, in vivo methylation assays showed that several proteins were methylated in the absence of protein synthesis. The incorporated methyl groups were alkali sensitive. Of special interest was the observation that the methylation level of a 51,000-Mr protein increased two- to fivefold upon addition of various sugar attractants and decreased after the removal of the attractants. The increase was less pronounced in mutants defective in sugar chemotaxis and appeared to be specifically involved with sugar chemotaxis. Furthermore, cell fractionation and in vitro methylation assays demonstrated that the 51,000-Mr protein is located in the cytoplasmic membrane. These results suggest that a specific methyl-accepting chemotaxis protein is involved in multiple-sugar chemotaxis by C gelida. During chemotaxis, the changes of methylesterase activity in C gelida cells were similar to those in Escherichia coli RP437 cells, as determined by a continuous-flow assay for methanol evolution. Thus, the mechanism of methyl-accepting chemotaxis protein-mediated chemotaxis of the gram-positive C. gelida appears to be similar to that of the gram-negative E. coli rather than to that of other gram-positive bacteria, such as Bacillus subtilis.

Amino acid efflux in response to chemotactic and osmotic signals in Bacillus subtilis

We observed a large efflux of nonvolatile radioactivity from Bacillus subtilis in response to the addition of 31 mM butyrate or the withdrawal of 0.1 M aspartate in a flow assay. The major nonvolatile components effluxed were methionine, proline, histidine, and lysine. In studies of the release of volatile radioactivity in chemotaxis by B. subtilis cells that had been labeled with [3H]methionine, the breakdown of methionine to methanethiol can contribute substantially to the volatile radioactivity in fractions following addition of 0.1 M aspartate. However, methanol was confirmed to be released after aspartate addition and, in lesser quantities, after aspartate withdrawal. Methanol and methanethiol were positively identified by derivitization with 3,5-dinitro-benzoylchloride. Amino acid efflux but not methanol release was observed in response to 0.1 M aspartate stimulation of a cheR mutant of B. subtilis that lacks the chemotaxis methylesterase. The amino acid efflux could be reproduced by withdrawal of 0.1 M NaCl, 0.2 M sucrose, or 0.2 M xylitol and is probably the result of changes in osmolarity. Chemotaxis to 10 mM alanine or 10 mM proline resulted in methanol release but not efflux of amino acids. In behavioral studies, B. subtilis tumbled for 16 to 18 s in response to a 200 mosM upshift and for 14 s after a 20 mosM downshift in osmolarity when the bacteria were in perfusion buffer (40 mosM). The pattern of methanol release was similar to that observed in chemotaxis. This is consistent with osmotaxis in B. subtilis away from an increase or decrease in the osmolarity of the incubation medium. The release of methanol suggests that osmotaxis is correlated with methylation of a methyl-accepting chemotaxis protein.

Chemotaxis in Bacillus subtilis: how bacteria monitor environmental signals

Virtually all organisms have means of monitoring their environment and making use of information gained to aid their survival. Many organisms, from bacteria to animals, move from place to place and can alter their movements. Chemotaxis is a signal transduction system found in motile bacteria that allows them to sense changes in the concentrations of various extracellular compounds and change their swimming behavior in a way that moves them toward more favorable environments. Chemotaxis is the most ancient sensory-motor process in nature. For years, studies of enteric bacteria, such as Escherichia coli and Salmonella typhimurium, have served as the paradigm for understanding this process on a molecular level. Recent studies on the gram-positive bacterium, Bacillus subtilis, and other bacteria, suggest that a slightly more complex system may be ancestral to that of the more extensively studied enterics. Aspects of chemotaxis that are unique to B. subtilis include a more complex adaptation system, with protein-protein methyl group transfer, chemotaxis proteins having no counterparts in E. coli, and a very extensive repertoire of repellents that are sensed at very low concentrations by receptors.

Altered chemotaxis of a Bacillus sphaericus L-ethionine-resistant sporulation mutant. A probable link between chemotaxis and sporulation

A UV irradiation-induced mutant of B. sphaericus 2362 whose sporulation was inhibited neither by natural amino acids nor by L-ethionine was selected. The mutant (A61) grew slowly in rich amino acid medium and contained increased concentrations of heat-resistant spores throughout the growth. Slow growth of A61 was related to continuous presence of aging and sporulating cells even when the medium was rich in nutrients. The ability of the mutant to sense nutrient presence in the environment and to relate this information to systems regulating the switch from vegetative growth to sporulation seem to be damaged. A61 also demonstrated impaired chemotaxis. In contrast to the parent strain, only few amino acids elicited chemotactic response in A61. Methylation of the A61 methyl-accepting chemotaxis protein(s) was lower than that of the parent strain by one order of magnitude. Spontaneous fast-growing phenotypic revertants of A61 displayed sporulation behavior characteristic of B. sphaericus 2362. Their chemotaxis to amino acids was considerably improved. To some amino acids, it proved to be even stronger than in the original strain, B. sphaericus 2362. It is suggested, that methyl transfer events originating in the chemotactic system are involved in the triggering of sporulation, the A61 mutation being located in this signalling pathway.

Chemotaxis, sporulation, and larvicide production in Bacillus sphaericus 2362. The influence of L-ethionine, and of aminophenylboronic acid

4-Aminophenylboronic acid (APBA), a known inhibitor of sporulation in Bacilli, as well as L-ethionine, a known inhibitor of chemotaxis in Enterobacteria, inhibited both sporulation and chemotactic behavior but not growth of Bacillus sphaericus. Both compounds also inhibited the methyl group turnover on the methyl-accepting chemotaxis protein (P53) in this microorganism. Sporulation of B. sphaericus was inhibited only when APBA was added to the growing culture before the late logarithmic stage. It was previously demonstrated that the ability of B. sphaericus to respond to chemoattractants sharply declines at the same age of the culture. Thus, it seems plausible that the action of both inhibitors upon sporulation may be attributed to the inhibition of some regulatory pathway common to chemotaxis and sporulation and involving protein methylation. Possible exchange of the nutrient depletion-related sensory information between chemotaxis and sporulation systems at the level of methyl group transfer is discussed.

Altered chemotaxis of Bacillus sphaericus L-ethionine-resistant sporulation mutant. A probable link between chemotaxis and sporulation

A UV irradiation-induced mutant of Bacillus sphaericus 2362 whose sporulation was inhibited neither by natural amino acids nor by L-ethionine was selected. The mutant (A61) grew slowly in rich amino acid medium and contained increased concentrations of heat-resistant spores throughout the growth. Slow growth of A61 was related to continuous presence of aging and sporulating cells even when the medium was rich in nutrients. Ability of the mutant to sense nutrient presence in the environment and to relate this information to systems regulating the switch from vegetative growth to sporulation seem to be damaged. A61 also demonstrated impaired chemotaxis. In contrast to the parent strain, only few amino acids elicited chemotactic response in A61. Methylation of the A61 methyl-accepting chemotaxis protein(s) was lower than that of the parent strain by one order of magnitude. Spontaneous fast-growing phenotypic revertants of A61 displayed sporulation behavior characteristic of B. sphaericus 2362. Their chemotaxis to amino acids was considerably improved. To some amino acids, it proved to be even stronger than in the original strain, B. sphaericus 2362. It is suggested, that methyl transfer events originating in the chemotactic system are involved in the triggering of sporulation, the A61 mutation being located in this signalling pathway.

Chemotaxis, sporulation, and larvicide production in Bacillus sphaericus 2362. The influence of L-ethionine, and of aminophenylboronic acid

4-Aminophenylboronic acid (APBA), a known inhibitor of sporulation in Bacilli, as well as L-ethionine, a known inhibitor of chemotaxis in Enterobacteria, inhibited both sporulation and chemotactic behavior but not growth of Bacillus sphaericus. Both compounds also inhibited the methyl group turnover on the methyl-accepting chemotaxis protein (P53) in this microorganism. Sporulation of B. sphaericus was inhibited only when APBA was added to the growing culture before the late logarithmic stage. It was previously demonstrated that the ability of B. sphaericus to respond to chemoattractants sharply declines at the same age of the culture. Thus, it seems plausible that the action of both inhibitors upon sporulation may be attributed to the inhibition of some regulatory pathway common to chemotaxis and sporulation and involving protein methylation. Possible exchange of the nutrient depletion-related sensory information between chemotaxis and sporulation systems at the level of methyl group transfer is discussed.

Influence of attractants and repellents on methyl group turnover on methyl-accepting chemotaxis proteins of Bacillus subtilis and role of CheW

The ability of attractants and repellents to affect the turnover of methyl groups on the methyl-accepting chemotaxis proteins (MCPs) was examined for Bacillus subtilis. Attractants were found to cause an increase in the turnover of methyl groups esterified to the MCPs, while repellents caused a decrease. These reactions do not require CheW. However, a cheW null mutant exhibits enhanced turnover in unstimulated cells. Assuming that the turnover of methyl groups on the MCPs reflects a change in the activity of CheA, these results suggest that the activation of CheA via chemoeffector binding at the receptor does not require CheW.

Bacillus subtilis CheN, a homolog of CheA, the central regulator of chemotaxis in Escherichia coli

The Bacillus subtilis cheN gene was isolated, sequenced, and expressed. It encodes a large negatively charged protein with a molecular weight of approximately 74,000. The predicted protein sequence has 33 to 34% identity with the Escherichia coli and Salmonella typhimurium CheA and Myxococcus xanthus FrzE sequences. These proteins are found to autophosphorylate and are members of the same histidine kinase signal modulating family. CheN has several conserved regions (including the histidine that is phosphorylated in CheA) that coincide with other autophosphorylated signal transducers. A null mutant is defective in attractant-induced methanol formation and shows no behavioral response to chemoeffectors. These results imply that in B. subtilis the mechanism of chemotaxis involves phosphoryl transfer similar to that in E. coli. However, the CheN null mutant mostly tumbles, whereas CheA mutants swim smoothly, and only in B. subtilis does excitation lead to methyl transfer and methanol formation. Thus, the overall mechanism of chemotaxis is different in the two organisms.

Identification and purification of a calcium-binding protein from Bacillus subtilis

A Ca(2+)-binding protein was identified in Bacillus subtilis in the log phase of growth. The molecular mass of this protein is about 38 kDa as estimated by polyacrylamide gel electrophoresis in the presence of SDS and by gel filtration. The protein was found to be resistant 10 min at 65 degrees C and was purified about 400 times, starting from heated crude extract, by conventional procedures. This novel protein is able to bind Ca2+ in the presence of an excess of MgCl2 and KCl both in solution and after SDS gel electrophoresis and electrotransfer. Since an impairment of the Ca2+ intake, in Bacillus subtilis, results in an impairment of chemotactic behavior (Matsushita, T. et al (1988) FEBS lett. 236, 437-440), 38 kDa protein may be involved in the regulation of chemotaxis.

The lifetime of photosensory signals in Halobacterium halobium and its dependence on protein methylation

Halobacteria spontaneously reverse their swimming direction about every 10 s. This behavioral pattern is transiently disturbed upon stimulation through sensory photosystems of different spectral sensitivity. As a result of stimulation, a single swimming interval is either prolonged (attractant response) or shortened (repellent response). Thereafter the cell returns to its autonomous reversal rhythm, i.e., it quickly adapts. Method are presented to determine the lifetime of repellent as well as of attractant cellular signals at the site of signal integration, using particular stimulation programs. Independent of the photosystem through which the signals were generated, the total lifetime of a repellent signal was 1.3 s. The decay of the signal was rapid during the first 100 ms and slow thereafter. The lifetime of an attractant signal was about 4 s and likewise did not depend on the photosystems. The degree of methylation of membrane proteins was increased by attractant stimuli and decreased by repellent stimuli. Inhibition of protein methylation by homocysteine was accompanied by a slowdown of the decay of both the repellent and attractant signal. A mutant strain with an increased demethylation also gave increased signal lifetimes. A lowered Ca2+ concentration, which activates methylation in vivo, led to shortened signal lifetimes. Methylation is proposed to be the mechanism which limits the signal lifetime and thereby allows the cells to adapt.

Transcriptional organization of a cloned chemotaxis locus of Bacillus subtilis

A cloned chemotaxis operon has been characterized. Thirteen representative che mutations from different complementation groups were localized on the physical map by recombination experiments. The use of integration plasmids established that at least 10 of these complementation groups within this locus are cotranscribed. An additional three complementation groups may form part of the same transcript. The direction of transcription and the time of expression were determined from chromosomal che-lacZ gene fusions. The promoter was cloned and localized to a 3-kilobase fragment. Expression of beta-galactosidase from this promoter was observed primarily during the logarithmic phase of growth. Three-factor PBS1 cotransduction experiments were performed to order the che locus with respect to adjacent markers. The cheF141 mutation is 70 to 80% linked to pyrD1. This linkage is different from that reported previously (G. W. Ordal, D. O. Nettleton, and J. A. Hoch, J. Bacteriol. 154:1088-1097, 1983). The cheM127 mutation is 57% linked by transformation to spcB3. The gene order determined from all crosses is pyrD-cheF-cheM-spcB.

Methyl transfer in chemotaxis toward sugars by Bacillus subtilis

Like amino acids, the sugars glucose and the nonmetabolizable 2-deoxyglucose caused a turnover of methyl groups on the methyl-accepting chemotaxis proteins. These sugars also caused methanol formation on addition. Thus, in contrast to chemotaxis in Escherichia coli, taxis to phosphotransferase sugars by Bacillus subtilis utilizes the methyl-accepting chemotaxis proteins.

Nucleotide sequence and expression of cheF, an essential gene for chemotaxis in Bacillus subtilis

The cheF gene, which is involved in chemotaxis in Bacillus subtilis, has been cloned, expressed, and sequenced. This gene is contained in a 0.7-kilobase PstI DNA fragment that was isolated from a lambda Charon 4A B. subtilis chromosomal DNA library. This fragment was subcloned into the expression vector pSI-1 and shown to complement the cheF mutation both for chemotaxis and for methanol production in response to the addition of attractants. Plasmid-encoded DNA expression in B. subtilis maxicells indicated that a membrane-associated polypeptide of 20-kilodaltons was expressed from this 0.7-kilobase DNA. The nucleotide sequence of this DNA fragment was determined, and an open reading frame capable of encoding a putative 175-amino-acid protein (Mr 20,002) was identified. In an effort to understand the function of the cheF protein, the dosage of the cheF gene product was varied by altering the concentration of IPTG (isopropyl-beta-D-thiogalactopyranoside) during growth. In the presence of high concentrations of IPTG, chemotaxis was inhibited and methanol production was impaired.

Functional homology of chemotactic methylesterases from Bacillus subtilis and Escherichia coli

The methylesterase enzyme from Bacillus subtilis was compared with that from Escherichia coli. Both enzymes were able to demethylate methyl-accepting chemotaxis proteins (MCPs) from the other organism and were similarly affected by variations in glycerol, magnesium ion, or pH. When attractants were added to a mixture of B. subtilis MCPs and E. coli methylesterase, the rate of demethylation was enhanced. Conversely, when attractants were added to a mixture of E. coli MCPs and B. subtilis methylesterase, the rate of demethylation was diminished. These effects are what would be expected if, in these in vitro systems, the MCPs determined the rate of demethylation. These data suggest that, although the enzymes are from evolutionarily divergent organisms and are different in size, they have considerable functional homology.

Evidence for methyl group transfer between the methyl-accepting chemotaxis proteins in Bacillus subtilis

We present evidence for methyl (as methyl or methoxy) transfer from the methyl-accepting chemotaxis proteins H1 and possibly H3 of Bacillus subtilis to the methyl-accepting chemotaxis protein H2. This methyl transfer, which has been observed in vitro (D. J. Goldman and G. W. Ordal, Biochemistry 23:2600-2606, 1984), was strongly stimulated by the chemoattractant aspartate and thus may play an important role in the sensory processing system of this organism. Although radiolabeling of H1 and H3 began at once after the addition of [3H]methionine, radiolabeling of H2 showed a lag. Furthermore, the addition of excess nonradioactive methionine caused immediate exponential delabeling of H1 and H3 while labeling of H2 continued to increase. Methylation of H2 required the chemotactic methyltransferase, probably to first methylate H1 and H3. Aspartate caused increased labeling of H2 and strongly decreased labeling of H1 and H3 after the addition of nonradioactive methionine. Without the addition of nonradioactive methionine, aspartate caused demethylation of H1 and to a lesser extent H3, with an approximately equal increase of methylation of H2.