[3 5S]Sulfate incorporation into myelin glycoproteins I. Central nervous system

The in vivo incorporation of [^{3 5}S]sulfate and [³H]fucose into rat brain myelin was investigated. Most of the ^{3 5}S in the myelin was in sulfatide, but about 4% was associated with the residual proteins after chloroform/methanol extraction. Polyacrylamide gel electrophoresis of these proteins indicated that the major ^{3 5}S-labeled component corresponded to the major fucose-labeled glycoprotein. The labeling of this predominant glycoprotein with sulfate was more selective than with fucose, since there was relatively little incorporation of sulfate into some of the minor fucose-labeled glycoproteins. There was little or no ^{3 5}S associated with proteolipid or basic protein on polyacrylamide gels. The fucose-labeled glycoproteins were converted to glycopeptides by pronase digestion and separated into two major classes by gel filtration on Sephadex-G50. Only the higher molecular weight class contained significant amounts of ^{3 5}S. The association of ^{3 5}S with the glycopeptides was not due to binding of sulfatide or free inorganic sulfate. The results indicate that the predominant myelin-associated glycoprotein in rat brain is sulfated.     

Sphingosylphosphorylcholine and sphingosinew-1-phosphate mobilize cytosolic calcium through different mechanisms in human airway epithelial cells

The sphingosine derivatives sphingosylphosphorylcholine (SPC) and sphingosine-1-phosphate (S1P) caused a similar elevation of the intracellular Ca²⁺ concentration ([Ca²⁺]_i) in an immortalized airway epithelial cell line (CFNP9o⁻) incubated in Ca²⁺-free medium. The maximal effect was obtained with 2 μM SPC and 0.1 μM S1 P and was sensitive to pre-incubation with pertussis toxin, indicating the involvement of a G_i/G_o type of G protein. In Ca²⁺ containing medium, [Ca²⁺]_i elevation by SPC was significantly higher than that by S1P, due to the fact that SPC was able to stimulate Mn²⁺ entry, whereas S1P was ineffective. SPC, but not S1P, caused a dose-dependent production of total inositol phosphates. Conversely, S1P, but not SPC, increased the level of phosphatidic acid. These findings suggest the presence of two distinct receptors, specific for SPC and S1P, respectively. Depletion of intracellular Ca²⁺ stores by SPC makes cells unable to respond to a subsequent addition of S1P. Conversely, cells do respond to SPC after a challenge with S1P, suggesting that the two receptors likely share one or more intracellular signalling component(s).     

Evidence for specific synaptosomal localization of exogenous accumulated taurine

Abstract— Equilibrium or incomplete equilibrium density gradient centrifugation was used to characterize the subcellular localization of exogenous [^{3 5}S]taurine which was taken up by minces or homogenates of rat cerebral cortex. [^{3 5}S]Taurine is accumulated in synaptosomes, which sediment more slowly than l -[³H]norepinephrine-accumulating particles. When [^{3 5}S]taurine and [³H]GABA are accumulated by minces, a small difference in the sedimentation profile of taurine and GABA was observed, but no difference was found when taurine and intrasynaptosomal potassium were compared. However, potassium sedimented more slowly after incubation of homogenates than of minces. These data give evidence for the accumulation of [^{3 5}S]taurine by a specific synaptosomal population.     

Protein-protein proximity in the association of ribosomal subunits of Escherichia coli: crosslinking of 30 S protein S16 to 50 S proteins by glutaraldehyde or formaldehyde

The interaction of ribosomal subunits from Escherichia coli has been studied using crosslinking reagents. Radioactive ³⁵S-labeled 50 S subunits and non-radioactive 30 S subunits were allowed to reassociate to form 70 S ribosomes. The 70 S particles, containing radioactivity only in the 50 S protein moiety, were incubated with glutaraldehyde or formaldehyde. As a result of this treatment a substantial fraction of the 70 S particles did not dissociate at 1 mm-Mg²⁺. This fraction was isolated and the ribosomal proteins were extracted. The protein mixture was analyzed by the Ouchterlony double diffusion technique by using eighteen antisera prepared against single 30 S ribosomal proteins (all except those against S3, S15 and S17). As a result of the crosslinking procedure it was found that only anti-S16 co-precipitated ³⁵S-labeled 50 S protein. It is concluded that the 30 S protein S16 is at or near the site of interaction between subunits and can become crosslinked to one or more 50 S ribosomal proteins.     

A proposed role for IF-3 and EF-T in maintaining the specificity of prokaryotic initiation complex formation

Initiation factor-free 30S subunits of E. coli ribosomes bind aminoacyl-tRNAs more efficiently than fMet-tRNA _inff ^supMet . Elongator-tRNA binding was unaffected by IF-1 or IF-2 but was inhibited by IF-3. Their combination reduced this binding up to 40% and stimulated that of fMet-tRNA _inff ^supMet . Unexpectedly, EF-T also prevented elongator-tRNA binding by complexing both to the 30S and to the aminoacyl-tRNAs. Using AUGU₃ as mRNA, elongator-tRNAs competed with fMet-fRNA _inff ^supMet and with tRNA _inff ^supMet . fMet-tRNA _inff ^supMet reacted with puromycin after addition of 50S subunits suggesting that it occupied the P site. EF-T directed binding of phe-tRNA to the 30S.AUGU₃ complex at the A site only if fMet-tRNA _inff ^supMet or tRNA _inff ^supMet filled the P/E site. We propose that one function of EF-T may be to prevent the entry of aminoacyl-tRNAs into the 30S particle during initiation. The possibility that a special site for fMet-tRNA resides on 16S rRNA is also discussed.     

Effects of Sulfhydryl Reagents on Uterine Nuclear Estrogen Receptors Labeled by in Vitro Exchange with [3H]Estradiol or [3H]4-Hydroxytamoxifen

Abstract

We have attempted to convert 4 S uterine nuclear estrogen receptors obtained after in vitro labeling with [³H]antiestrogens to 3 S, the form observed after in vitro exchange with [³H]estradiol, in order to examine the possible relationship between these forms. Treatment of nuclear extracts labeled with the high affinity antiestrogen, [³H]4-hydroxytamoxifen, with a variety of nucleases, phosphatases, or proteases either had no effect on the 4 S antiestrogen-receptor complex or led to loss of ligand binding. The sulfhydryl reducing agents, cysteine or reduced glutathione, on the other hand, brought about conversion of 4 S estrogen receptors to components sedimenting at about 3 S. Conversely, when oxidized glutathione was included in all buffers used for preparation and labeling of nuclear estrogen receptors with [³H]estradiol, more rapidly sedimenting (?4.6 S) forms of estrogen-receptor complex predominated. Cysteine still effected the 4 S to 3 S conversion when nuclear estrogen receptors, partially purified by sucrose gradient centrifugation, were used as substrate, suggesting a direct action of the sulfhydryl reagents on receptor molecules. From these results we propose that nuclear estrogen and antiestrogen-receptor complexes may differ in conformation such that the former may be more sensitive to the action of an endogenous reducing agent which contributes to formation of 3 S [³H]estradiol-receptor complexes.     

Biogeochemical cycle of Sulfer in the Calamagrostis angustifolia wetland ecosystem in the Sanjiang Plain, China

To better understand the Sulfur (S) cycle in the wetland ecosystem, the S cycle and its compartmental distribution within an atmosphere-plant-soil system were studied using a compartment model in the Calamagrostis angustifolia wetland in the Sanjiang Plain, Northeast China. The results showed that the soil was the main S storage and flux hinge in which 97.78% S was accumulated. In the plant subsystem, the root was the main S storage, and it remained at 79.60% of the total S contents, which in the Calamagrostis angustifolia wetland ecosystem showed that the parts above the ground took up 0.75 g S/m², the S re-transferring biomass to the root was 0.24 g S/m², and to the litter was 0.51 g S/m²; the root took up 3.76 g S/m² and the S transferring biomass to the soil took up 3.07 g S/m²; the litter S biomass was 0.75 g S/(m²·a) and the S transferring biomass to the soil was more than 0.52 g S/(m²·a). The emission amount of H2S from the Calamagrostis angustifolia wetland ecosystem to the atmosphere was 1.42 mg S/m², whereas carbonyl sulfide (COS) was absorbed by the Calamagrostis angustifolia wetland from the atmosphere and the absorption amount was 1.83 mg S/m². The S input biomass from the rain to the ecosystem was 4.85mg S/m² during the growing season. The difference between input and output amounts was 5.26 mg S/m², which indicated that S was accumulated in the ecosystem and would lead to wetland acidification in the future.     

Importance of sulfate,cysteine and methionine as precursors to felinine synthesis by domestic cats (Felis catus)

There is conflicting evidence in the literature on the utilization of cysteine and methionine as precursors to the urinary sulfur-containing amino acid felinine in cats. Three entire domestic short-haired male cats, housed individually in metabolism cages, were injected intraperitoneally with either [³⁵S]-sulfate, [³⁵S]-cysteine, or [³⁵S]-methionine. Daily urine samples were collected quantitatively for up to 9 days after injection. Each cat was injected once with each compound after observing an appropriate interval for [³⁵S] to be depleted between injections. All the urine samples were analysed for felinine content and total radioactivity. Felinine was isolated from each urine sample and analysed for radioactivity. No radioactivity was found in felinine from cats injected with [³⁵S]-sulfate. The mean (±S.E.M.) cumulative recovery of radioactivity in the urine of the [³⁵S]-sulfate injected cats was 90.6±6.1% after 4 days. The mean (±S.E.M.) cumulative incorporation rate of radioactivity into felinine by the cats receiving the [³⁵S]-cysteine and [³⁵S]-methionine were 11.6±1.6 and 8.6±0.6%, respectively, after 9 days. The mean (±S.E.M.) cumulative recoveries of radioactivity in the urine were 58.1±3.7 and 36.0±8.0%, respectively. Cysteine and methionine, but not sulfate, are precursors to felinine, with cysteine being a more quantitatively important precursor compared to methionine.     

Biogeochemical cycle of Sulfer in the Calamagrostis angustifolia wetland ecosystem in the Sanjiang Plain, China

To better understand the Sulfur (S) cycle in the wetland ecosystem, the S cycle and its compartmental distribution within an atmosphere-plant-soil system were studied using a compartment model in the Calamagrostis angustifolia wetland in the Sanjiang Plain, Northeast China. The results showed that the soil was the main S storage and flux hinge in which 97.78% S was accumulated. In the plant subsystem, the root was the main S storage, and it remained at 79.60% of the total S contents, which in the Calamagrostis angustifolia wetland ecosystem showed that the parts above the ground took up 0.75 g S/m², the S re-transferring biomass to the root was 0.24 g S/m², and to the litter was 0.51 g S/m²; the root took up 3.76 g S/m² and the S transferring biomass to the soil took up 3.07 g S/m²; the litter S biomass was 0.75 g S/(m²·a) and the S transferring biomass to the soil was more than 0.52 g S/(m²·a). The emission amount of H2S from the Calamagrostis angustifolia wetland ecosystem to the atmosphere was 1.42 mg S/m², whereas carbonyl sulfide (COS) was absorbed by the Calamagrostis angustifolia wetland from the atmosphere and the absorption amount was 1.83 mg S/m². The S input biomass from the rain to the ecosystem was 4.85mg S/m² during the growing season. The difference between input and output amounts was 5.26 mg S/m², which indicated that S was accumulated in the ecosystem and would lead to wetland acidification in the future.     

Involvement of ethylene in reversal of salt‐inhibited photosynthesis by sulfur in mustard

Sulfur (S) assimilation results in the synthesis of cysteine (Cys), a common metabolite for the formation of both reduced glutathione (GSH) and ethylene. Thus, ethylene may have regulatory interaction with GSH in the alleviation of salt stress. The involvement of ethylene in the alleviation of salt stress by S application was studied in mustard (Brassica juncea cv. Pusa Jai Kisan). First, the effects of 0, 0.5, 1.0 and 2.0 mM SO₄^2? were studied on photosynthetic and growth parameters to ascertain the S requirement as sufficient‐S and excess‐S for the plant. In further experiments, the effects of sufficient‐S (1 mM SO₄^2?) and excess‐S (2 mM SO₄^2?) were studied on the alleviation of salt stress‐induced by 100 mM NaCl, and ethylene involvement in the alleviation of salt stress by S. Under non‐saline condition, excess‐S increased ethylene with less content of Cys and GSH and adversely affected photosynthesis and growth. In contrast, excess‐S maximally alleviated salt stress due to high demand for S and optimal ethylene formation, which maximally increased GSH and promoted photosynthesis and growth. The involvement of ethylene in S‐mediated alleviation of salt stress was further substantiated by the reversal of the effects of excess‐S on photosynthesis by aminoethoxyvinylglycine (AVG), ethylene biosynthesis inhibitor. The studies suggest that plants respond differentially to the S availability under non‐saline and salt stress and excess‐S was more potential in the alleviation of salt stress. Further, ethylene regulates plants' response and excess S‐induced alleviation of salt stress and promotion of photosynthesis.     

Effect of high boron application on boron content and growth of melons

Synthetic chelates, such as ethylene diamine tetraacetic acid (EDTA), have been shown to enhance phytoextraction of Pb from contaminated soil but also cause leaching of heavy metal-chelate complexes, posing a groundwater contamination threat. In a soil column study, we examined the effect of EDTA and a biodegradable chelate [S,S] isomere of ethylene diamine disuccinate ([S,S]-EDDS), newly introduced in phytoextraction research, on the uptake of Pb by the Chinese cabbage (Brassica rapa) and Pb leaching through the soil profile. Soil water sorption characteristics were modified by acrylamide hydrogel. The addition of 0.1 and 0.2% (w/w) of hydrogel amendments increased soil field water capacity from initial 24.6% to 28.5% and 31.3%, respectively. The additions of 2.5, 5 and 10 mmol EDTA kg^–1 soil were more effective in enhancing Pb plant uptake than comparable [S,S]-EDDS treatments, but caused (as also 10 mmol kg^–1 [S,S]-EDDS additions) unacceptably high Pb leaching in treatments with any soil water sorption conditions tested. The most efficient level of EDTA (10 mmol kg^–1) enhanced plant Pb uptake by 97 times compared to the control. Shoots Pb concentrations reached 500 mg kg^–1 of dry biomass. However, in this treatment 36.2% of total initial Pb was leached from the soil during the first four weeks after chelate addition. Hydrogel soil amendments were more effective in treatments with [S,S]-EDDS than with EDTA. In treatments with 10 mmol kg^–1[S,S]-EDDS hydrogel amended soils, plant Pb uptake was significantly reduced and Pb leach was as high as 44.2% of total initial soil Pb. At lower [S,S]-EDDS concentrations, the effect of hydrogel soil amendment on Pb leaching was the opposite. The addition of 5 mmol kg^–1 [S,S]-EDDS soil to the soil amended with 0.2% hydrogel increased Pb uptake by 18 times while only 0.2% of total initial Pb was leached. In all treatments, the concentrations of Pb in dry plant biomass were far from concentrations required for efficient soil remediation within a reasonable time span.     

Methylation of ribosomal proteins in HeLa cells.

Methylated amino acids from both 40 and 60S subunit proteins of HeLa cytoplasmic ribosome were analyzed. It was observed that methylation of ribosomal proteins occurs in both subunits with N^G,N^G-dimethylarginine as the major methylated amino acid. The presence of N^G,N^G-dimethylarginine has been identified by high-voltage paper electrophoresis, by paper chromatography, and by amino acid analysis. In addition, both ribosomal subunits contain methylated lysines with ?-N-trimethyllysine being the predominant one, followed by ?-N-dimethyllysine. Little, if any ?-N-monomethyllysine was detected in either subunit. The cytoplasmic 60S ribosomal subunit contains much more ?-N-trimethyllysine compared to the 40S ribosomal subunit. The possible biological significance of methylation was discussed.     

The stereochemical course of thiophosphoryl group transfer catalyzed by adenosine kinase

Adenosine kinase was partially purified form beef liver and used to catalyze the conversion of (γR)ATPγS,γ¹⁸O and adenosine to ADP and AMPαS,α¹⁸O. The configuration at phosphorus in AMPαS,α¹⁸O was established by subjecting it to stereospecific phosphorylation to (αS)ATPαS,α¹⁸O and showing that only the nonbridging oxygen bonded to the α-P was enriched with ¹⁸O. The configuration at α-P in AMPαS,α¹⁸O was therefore S, and the transfer of the [¹⁸O]thiophosphoryl group occurred with $\text{inversion}$ of configuration.     

Inhibition of nucleic acid synthesis in Ehrlich tumor cells by periodate-oxidized adenosine and adenylic acid

Periodate-oxidized adenosine and AMP were inhibitory to both RNA and DNA synthesis in Ehrlich tumor cells in culture. With periodate-oxidized adenosine, the inhibition of RNA synthesis paralleled the inhibition of DNA synthesis. Periodate-oxidized AMP, however, was more inhibitory to DNA synthesis than to RNA synthesis. With both compounds, there was a decrease in the conversion of [¹⁴C]cytidine nucleotides to [¹⁴C]deoxycytidine nucleotides in the acid-soluble pool. The borohy-dride-reduced trialcohol derivative of the periodate-oxidized adenosine compound was not inhibitory to DNA or RNA synthesis in the tumor cells. The incorporation of [³H]uridine into 28S and 18S ribosomal RNA was inhibited by both periodate-oxidized adenosine and AMP, but the incorporation of [³H]uridine in 45S, 5S, and 4S RNA was essentially unaffected by these compounds. Periodate-oxidized adenosine inhibited Ehrlich tumor cell growth in vivo.     

[2-3H]ATP synthesis and 3H NMR spectroscopy of enzyme-nucleotide complexes: ADP and ADP.Vi bound to myosin subfragment 1

Summary The synthesis of [2-³H]ATP with specific activity high enough to use for ³H NMR spectroscopy at micromolar concentrations was accomplished by tritiodehalogenation of 2-Br-ATP. ATP with greater than 80% substitution at the 2-position and negligible tritium levels at other positions had a single ³H NMR peak at 8.20 ppm in 1D spectra obtained at 533 MHz. This result enables the application of tritium NMR spectroscopy to ATP utilizing enzymes.The proteolytic fragment of skeletal muscle myosin, called S1, consists of a heavy chain (95 kDa) and one alkali light chain (16 or 21 kDa) complex that retains myosin ATPase activity. In the presence of Mg²⁺, S1 converts [2-³H]ATP to [2-³H]ADP and the complex S1.Mg[2-³H]ADP has ADP bound in the active site. At 0°C, 1D ³H NMR spectra of S1.Mg[2-³H]ADP have two broadened peaks shifted 0.55 and 0.90 ppm upfield from the peak due to free [2-³H]ADP. Spectra with good signal-to-noise for 0.10 mM S1.Mg[2-³H]ADP were obtained in 180 min. The magnitude of the chemical shift caused by binding is consistent with the presence of an aromatic side chain being in the active site. Spectra were the same for S1 with either of the alkali light chains present, suggesting that the alkali light chains do not interact differently with the active site. The two broad peaks appear to be due to the two conformations of S1 that have been observed previously by other techniques. Raising the temperature to 20 °C causes small changes in the chemical shifts, narrows the peak widths from 150 to 80 Hz, and increases the relative area under the more upfield peak. Addition of orthovanadate (V_i) to produce S1.Mg[2-³H]ADP.V_i shifts both peaks slightly more upfield without chaning their widths or relative areas.     

Equilibration of [3H]glucosamine and [35S]sulfate with intracellular pools of UDP-N-acetylhexosamine and 3′-phosphoadenosine-5′-phosphosulfate (PAPS) in cultured fibroblasts

Nucleotides and sugar nucleotides were extracted from cultures of human fibroblasts with perchloric acid, separated by isotachophoresis, and quantified by uv absorption analysis at 254 nm. ATP (936 pmol/μg DNA) was, as expected, the dominating nucleotide pool. The energy charge was estimated to 0.9. The UDP-N-acetylhexosamine pool was also a very prominent compound (596 pmol/μg DNA). After incubation of fibroblasts with [³H]glucosamine, more than 95% of the acid-soluble radioactivity was found in the UDP-N-acetylhexosamine pool. Incubation with [³⁵S]sulfate resulted in the incorporation of [³⁵S]sulfate into 3′-phosphoadenosine-5′-phosphosulfate (PAPS). The latter could, however, only be measured as radioactivity, as the amount was too small to be quantified as total mass. Pulse-labeling of fibroblasts with [³⁵S]sulfate and [³H]glucosamine from 5 min to 16 h showed that [³⁵S]PAPS was equilibrated in less than 10 min, while [³H]glucosamine required a longer time, 2–4 h, to attain a steady state with UDP-N-acetylhexosamine. [¹⁴C]Glucose required approximately the same time as [³H]glucosamine to reach steady state with UDP-acetylhexosamine, which suggests that the reason for the long equilibration time is the slow turnover of this pool.     

Sulfur in Ghanain soils

Sulfur availability in twenty selected surface soils (0–22 cm), which varied in both physical and chemical properties and sampled under cultivated and uncultivated management in the various ecological zones of Ghana, was studied. Texture varied from coarse sand to clay, with 16–85% sand and 10–51% clay. Organic C varied from 0.45 to 2.24% and total N from 0.034 to 0.215%; soil pH (0.01M CaCl₂) from 3.69 to 7.43 and total S from 44 to 273 ppm. Inorganic sulfate formed 2.3 to 14.8% of the total S, HI-reducible S 4.4 to 28.2, C-bonded S 4.4 to 28.2 and unidentified organic S 12.7 to 63.2%. Sulfur availability was assessed by chemical extraction methods and electroultrafiltration technique as follows: (i) extraction with Ca(H₂PO₄)₂·H₂O solution containing 500 ppm P, (ii) extraction with 0.1M LiCl and (iii) electroultrafiltration (EUF) at 80°C, 400 V for 10 min and also on seven of the soils the standard EUF fractionation procedure of Neméth. Ca(H₂PO₄)₂-extractable S was not significantly correlated with LiCl-extractable S nor with any of the EUF values. LiCl-extractable S was not significantly correlated with sulfate extractable by and EUF?1+2+3 fractions (r=0.911^**). Dry matter yield of oat seedlings and EUF?1+2+3 fractions (r=0.911^**). Dry matter yield of oat seedlings was not correlated with any of the availability indexes. Total S uptake was significantly correlated with LiCl-extractable S (r=0.629^** without S and 0.729^** with S applied) and with EUF-80°C, 400 V/10 min (r=0.561^**), EUF-1 (r=0.953^***) and EUF-2 (r=0.912^**). On all the soils, more S was taken up by oat plants than could be accounted for by the inorganic S and S mineralized from organic S during an incubation period of 4 weeks.     

Axonal transport of 4S RNA in the chick optic system

The axonal transport of tRNA has been investigated in the chick optic system. Chicks were injected with [³H]uridine intraocularly or intracranially and the RNA of the retina, nerve complex, and tecta separated by polyacrylamide gel electrophoresis and then counted. The ratio of TRNA to rRNA specific activities increased with time in both the nerve complex and contralateral tectum. The ratio increased more rapidly in the nerve complex than the tectum. However, no increase was observed in the case of intracranially injected animals. This is consistent with the axonal flow of tRNA. When [methyl-³H]methionine was used as precursor, the preferential labeling of 4S RNA to rRNA which resulted more clearly showed a transport of 4S RNA from the retinal cells to the tectum. In conclusion, it was found that about 40% of the radioactive RNA observed within the optic tectum 4 days after an intraocular injection of [³H]uridine was accounted for by 4S RNA which had flowed from the retina. However, the migration of a methylated RNA molecule of size 4S, but unrelated to tRNA, cannot be entirely eliminated.     

Varietal differences in uptake of 32P labelled phosphate in clover plus ryegrass swards and monocultures

Lolium perenne cv. S.23, L. multiflorum cv. RvP, and Trifolium repens cvs S.184 and Olwen, were grown in mixed sward and monoculture during 1979. Whereas in mixtures grass roots absorbed more ³²P than clover roots, in monoculture clover generally absorbed more ³²P than grass roots. This showed that grass was a very strong competitor for uptake in mixed swards. Clover and grass monocultures absorbed most ³²P from 10 or 15 cm depth in the soil, while grass in mixtures absorbed most ³²P at 22.5 cm depth. Comparing varieties, in monocultures in June, Olwen was most active in absorbing ³²P at 15 cm. In August, Olwen absorbed more at 15 cm and 22.5 cm than S.184 or the grass varieties. Differences in absorption depth between varieties were less in mixtures than in monocultures. S.23 absorbed more ³²P in the late season than RvP, both in monoculture and in mixtures. Thus Olwen differed from S. 184 in depth and timing of uptake, whilst S.23 differed from RvP in time of uptake. Such varietal differences could be exploited by manipulation of depth and timing of fertiliser application to increase the precision of sward management.     

Uptake and assimilation of sulphate by sulphur deficient Zea mays cells: The role of O-acetyl-L-serine in the interaction between nitrogen and sulphur assimilatory pathways

Cell suspension cultures of maize (Zea mays) growing on a modified Murashige and Skoog's (MS) medium containing 1/10 of the normal N supply, were subjected to SO₄^2– starvation for 4 d. During the period of the experiment, the batches of cells were growing at similar rates in both +S and –S treatments. S-starved cells (–S) took up SO₄^2– at eight to ten times the rate of S-sufficient (+S) cells. The high uptake rate of –S cells was repressed within 1–2 h after SO₄^2– was re-supplied. The response to S-starvation was strongly diminished in cells which had been deprived of a N-source for 4 d. Cells grown for several culture cycles with homocysteine thiolactone (TL) as sole S-source had greatly increased SO₄^2– uptake rates. This enhanced uptake was repressed at similar rates by provision of SO₄^2– or by the renewal of the TL supply. The latter result was unexpected and cannot be explained at present. ATP-sulphurylase (EC 2.7.7.4) activity was also de-repressed: in –S cells, the measured activity being more than four times that in +S cells. Repression by SO₄^2– was observed although after a longer period than that for the repression of SO₄^2– uptake. In +S cells, SO₄^2– uptake and ATP-sulphurylase activity were increased significantly by the addition of 0.5 mol.m^–3 O-acetyl-L-serine to the culture. Simultaneously, the cysteine pool increased in the same proportion as the former activities. The addition of other amino acids, viz. glutamine or alanine, had either negative effects or no effect on SO₄^2– uptake.