Ontogenetic Change of the PSI- and PSII-Distribution in the Thylakoid System of Euglena gracilis

The thylakoid membranes of isolated Euglena chloroplasts wereseparated into two fractions by aqueous two-phase-partitioning(mixture of dextran 500 and poly(ethylene glycol) 4000) followingpress disruption. These two fractions differ in many respectsduring most of the cell cycle of this alga in comparison withthe thylakoid characteristics of higher plants or green algae.The amount of thylakoid membranes with separation characteristicscomparable with inside-out-vesicles of higher plant chloroplastschanges depending on the cell cycle stage of Euglena gracilis.Photosystems II and I are not restricted to one fraction. Boththylakoid membrane fractions evolve oxygen photosynthetically.When chloroplast differentiation in Euglena gracilis is complete(i.e. at the end of the light-time) the composition and thephotosynthetic efficiency of the two thylakoid fractions aregenerally equal. Photosystem I-related LHCI is present in bothfractions. Photosystem II-related CP29, however, was only detectedin unfractionated thylakoid membranes. The implications forthylakoid organization in Euglena chloroplasts are discussed. Key words: Euglena gracilis, photosystem I, photosystem II, stacking, thylakoids     

Oxidative Damages and Repair in Euglena gracilis Exposed to Ozone. I. SH Groups and Lipids

Malondialdehyde, a product of lipid oxidation, increased graduallywhen Euglena gracilis cells were bubbled with 240 µ1.liter^–1ozone (delivery rate of 1µmolO₃.min^–1) for 120 min.Simultaneously, the sulfhydryl group content decreased by 36%during the treatment, which was mainly due to oxidation of proteinsulfhydryl groups. The molar amount of SH groups oxidized was3 times higher than that of fatty acid oxidized, indicatingthat sulfhydryl groups were more accessible or more easily oxidizedby O₃ than fatty acids. When Euglena cells were allowed to recoverunder autotrophic growth conditions following O₃ treatment,viable cells were incapable of dividing during the first 5 hof the recovery period but regenerated SH groups nearly to thecontrol level. The increase of SH content during this periodpreceded the resumption of cell division and the restorationof normal growth. These results suggest that the regenerationof SH groups by Euglena cells is a part of a mechanism involvedin the repair of oxidative damage caused by ozone and is anessential step for the initiation of cell division. (Received July 20, 1987; Accepted December 14, 1987)     

Accumulation of Inorganic Carbon in the Cells of Euglena gracilis Z Grown Photoautotrophically in Ordinary Air

Affinity for inorganic carbon in photosynthesis of Euglena gracilisZ was higher in the cells grown in ordinary air than in thosegrown in 4% CO₂. The cells grown in ordinary air accumulatedinorganic carbon in the cells to the level far in excess ofthat expected from passive diffusion due to pH gradient acrossthe cell surface membrane. (Received April 5, 1986; Accepted June 25, 1986)     

The effect of UV-B radiation and humic substances on growth and motility of the flagellate, Euglena gracilis

The effects of ultraviolet-B radiation (UV-B, 250–315nm) were determined on Euglena gracilis with respect to speed,phototactic orientation, specific growth rate, and in the presenceof humic substances. Humic substances had a protective effectwhen studying the speed and specific growth rate. However, thedegree of phototactic orientation decreased in UV-B radiationboth with and without humic substances. The inhibition of O₂evolution and speed was most pronounced when using cutoff filtersWG280 and WG295. The photosynthetic inhibitor DCMU (10^–6M) did not have any effect on the speed, but the 0₂ evolutiondecreased to zero. The effect of different wavelengths in theUV-B region on the speed of E.gracilis showed the maximum sensitivityat 280 and 290 nm.     

Formation and division of giant mitochondria during the cell cycle of Euglena gracilis Z in synchronous culture I. Some characteristics of changes in the morphology of mitochondria and oxygen-uptake activity of cells

Changes in the morphology of mitochondria of Euglena gracilisZ cells were followed with an electron microscope during thecell cycle in a synchronous culture under photoautotrophic conditions.Giant mitochondria were temporarily formed, most probably byfusion of smaller forms, in the cells at an intermediate stagein the growth phase of the cell cycle. Formation of the giantmitochondria was accompanied by a striking decrease in the oxygen-uptakeactivity of the cells, and the division of giant mitochondriainto smaller forms by a re-increase in the activity. ¹ This work was reported in part at the 28th Annual Meetingof the Japanese Society of Electron Microscopy, May 1972. (Received November 8, 1973; )     

Methylation of Xenobiotic Thiols by Euglena gracilis: Characterization of a Cytoplasmic Thiol Methyltransferase

The green alga Euglena gracilis contains a thiol methyltransferasethat catalyzes the S-adenosylmethionine-dependent methylationof pentachlorobenzenethiol. The enzyme was localized in thecytoplasm and partially purified. The pH optimum for the enzymewas 6.5. The enzyme methylated a number of foreign thiols, butnot the cellular thiols, glutathione or cysteine. Phenols andanilines were not substrates. When pentachloro-benzenethiolwas the methyl acceptor the K_m was found to be 82 µM andthe corresponding K_m for S-adenosylmethionine was 140 µM.The molecular weight of the enzyme was 21,000, as determinedby gel filtration. A role for this enzyme in detoxifying xenobioticthiols is proposed. (Received September 28, 1984; Accepted April 25, 1985)     

Synthesis of Receptor Sites for Vitamin B12 in Euglena gracilis

In Euglena gracilis, vitamin B₁₂ uptake follows a biphasic patternconsisting of an initial rapid phase followed by a slower secondaryphase. Chase experiments showed that vitamin B₁₂ was tightlybound to its receptor-sites during either rapid or slow phaseof uptake. The slow phase was markedly inhibited when Euglenacells were preincubated with cycloheximide for 30 min at a concentrationof 100µg/ml. When the preincubation time was longer than30 min, a gradual inhibition of the rapid phase occurred andreached 84% after 4 h. This inhibitory effect of cycloheximideis reversible. On the other hand, tunicamycin at 1 µ/mlirreversibly inhibited the B₁₂ uptake suggesting that the receptor-sitefor B₁₂ is glycoprotein in nature. These results suggest thatthe rapid phase is also dependent on protein synthesis and representsB₁₂ binding on preexisting free receptors whereas the secondaryslow phase represents B_l2 binding on newly synthesized receptors.Both phases of uptake seem to be controled by the same receptor.The half-life of the free receptor is estimated to be 66 minwhereas the B₁₂ receptor complex is very stable. (Received October 3, 1988; Accepted February 15, 1989)     

Purification and Immunochemical Characterization of NADP-Dependent Glyceraldehyde 3-Phosphate Dehydrogenase of Euglena gracilis

NADP-dependent glyceraldehyde 3-phosphate dehydrogenase fromEuglena gracilis (EC 1.2.1.13 [EC] ) was purified about 170-fold bya two-step procedure involving DEAE-SH cellulose chromatographyand affinity chromatography on ADP-Sepharose. The homogeneousenzyme from mildly sonicated cells contained equal amounts oftwo types of subunits with mol wts of 34,000 (A) and 38,000(B). The active enzyme had a mol wt 144,000 and is thereforean A₂B₂ tetramer. Enzyme from strongly sonicated Euglena cellscontained, in addition, a second allomer with a probable A₄structure. NADdependent glyceraldehyde 3-phosphate dehydrogenase,a tetramer with 36,000 mol wt subunits, was unrelated immunologicallyto the NADP-dependent enzyme although the latter also showedminor NAD-dependent activity. Both isoenzymes of the NADPlinkedglyceraldehyde 3-phosphate dehydrogenase, however, were immunologicallyidentical. ¹Dedicated, to Prof. Dr. O. H. Volk on his 80th birthday. (Received October 13, 1982; Accepted March 21, 1984)     

Chemical Modification of the Membrane-bound ATP Synthetase of Spinach Chloroplasts with Pyridoxal Phosphate in Light

Photosynthetic activities of the thylakoid membranes modifiedwith pyridoxal phosphate (PLP) and sodium borohydride in lightwere studied and compared with those modified in the dark. PLPmodified the membrane-bound chloroplast coupling factor 1 (CF₁)and inhibited photophosphorylation. Only PLP modification inlight stimulated basal electron transport. This stimulationof electron transport was prevented by the presence of ATP orcarbonylcyanide m-chlorophenylhydrazone in the modificationmixture. Magnesium ion was required for PLP modification. Theextent of lightinduced proton uptake was decreased by PLP modificationin light. N,N'-Dicyclohexylcarbodiimide lowered the stimulatedelectron transport to the basal level of unmodified chloroplastsand restored proton uptake. When chloroplasts were modified with 4 mM PLP in light and dark,11.6 and 11.0 mol of PLP were incorporated into mol of CF₁,respectively. ATP could bind with high affinity to CF₁ isolatedafter PLP modification in light. The results indicate that PLP modifies membrane-bound CF₁ whichhas a conformation altered by energization of the thylakoidsin light, and causes an apparent uncoupling of phosphorylation(stimulation of basal electron transport). The results suggestthat this uncoupling is induced by the loss of the regulatoryfunction of CF₁ for proton translocation after PLP modificationin light. ¹ Presented at the ISRACON on Control Mechanisms in Photosynthesis.Aug. 31-Sept. 4, 1980, Acre, Israel (Received June 22, 1981; Accepted August 28, 1981)     

Cell Cycle Changes in the Shoot Apex of Silene coeli-rosa During the Second and Third Days of Floral Induction

The cell cycle in Silene coeli-rosa shoot apices was measuredto test whether or not early components of the floral stimulus,produced during the 2nd and 3rd long days (LD) of an inductiveLD treatment, resulted in an increase in the duration of G₂phase in constant 20–24 h cell cycles. Plants were grownat 20°C in short days (SD) of 8 h light and 16 h darknessfor 28 d (day 0). Starting on day 0, plants were given SD or3 LD each comprising an identical 8 h day and 16 h photo-extension,or 3 dark-interrupted (d.i.) non-inductive LD, interrupted at1700 h of each day with 1 h of darkness. The cell cycle (percentagelabelled mitoses method) and changes in cell number were determinedin the shoot apical meristem. During days 1–2 of the SDtreatment, the cell cycle and mean cell generation time (MCGT)was 18 and 32 h, respectively, giving a growth fraction of 56%.During days 2–3, the cell cycle and MCGT shortened to15 and 23 h, respectively (growth fraction = 65%). During days1–2 of the LD and d.i. LD treatments, cell cycles andMCGTs were 9–10 and 27–29 h, respectively, resultingin smaller growth fractions (about 33%). Thus, shortened cellcycles and altered growth fractions occurred regardless of whetheror not the treatment was inductive. The LD treatment resultedin a marked shortening of G₁ and, to a lesser extent, S-phase,whilst G₂ remained constant. These changes were consistent withincreases in the proportion of cells in G₂ during the photoextensionof each LD which were suppressed during the comparable periodsof the d.i. LD treatment. The latter treatment resulted in eachphase occupying virtually identical proportions of the cellcycle as in the SD treatment. Thus, the unique cell cycle responsesto the initial part of the inductive LD treatment were increasesin the proportion of cells in G₂ coupled with G₁ and G₂ beingof similar duration. Cell cycle, mean cell generation time, shoot apex, Silene coeli-rosa     

PARTIAL PHOTO-REPRESSION OF THE GLYOXYLATE BY-PASS IN EUGLENA

1. Addition of exogenous acetate or ethanol to autotrophic culturesof Euglena gracilis strain Z induces formation of the glyoxylateby-pass.
2. Visible light decreases the activity of malate synthasein greenEuglena by about 50%. No such effect was found in apermanentlybleached mutant.
3. Aconitase activity parallelsthat of malate synthase, but isocitricdehydrogenase activityis constant under all conditions examined.
4. Oxygen consumptionis proportional to the activities of malatesynthase and aconitase,but not to that of isocitric dehydrogenase.
5. The results ofsimilar studies with other growth substrates(pyruvate, malate,succinate) suggest that some of the oxygenconsumed by C₂-grownEuglena may not be associated with energyproduction.

(Received March 25, 1966; )     

Preparation of a Photoactive Reaction Center Complex Containing Photo-reducible Fe-S Centers and Photooxidizable Cytochrome c from the Green Sulfur Bacterium Chlorobium tepidum

A photoactive reaction center (RC) complex was isolated fromthe green sulfur bacterium Chlorobium tepidum by solubilizationof membranes with Triton X-100, followed by sucrosedensity gradientcentrifugation, DEAE Bio-Gel A chromatography, and hydroxyapatitechromatography. The purified RC complex contained about 50–70bacteriochlorophyll molecules (BChl) per P840, as assayed byphotooxidafion. It showed a near-infrared BChl a absorptionpeak at 814 nm and shoulders at about 800 and 835 nm at roomtemperature. SDS-PAGE analysis revealed 6 polypeptides withapparent molecular masses of 100, 65, 41, 32, 23, and 18 kDa.The RC complex binds functional P840 and Cyt c₅₅₁, which werephotooxidized by continuous illumination at room temperature.Upon flash excitation, the bound Cyt c₅₅₁ was oxidized, andrereduced in the dark with a half-time of 16 and 400 ms in thepresence and absence of 0.1 mM 2,6-dichlorophenol indophenol,respectively, at room temperature. At 551 nm, the amount ofthe Cyt c photooxidized by continuous illumination was 60% ofthe amount determined by chemical oxidation-reduction. The functionalCyt c₅₅₁/P840 ratio was calculated to be 1.2–1.7. EPRspectroscopy at cryogenic temperatures revealed that the RCcomplex binds three photoreducible Fe-S centers designated tobe CF_A, CF_B and CF_X (C for Chlorobium). CF_A and CF_B were reducedin the dark with dithionite at pH 10. (Received May 26, 1993; Accepted October 4, 1993)     

Oxidative Damages and Repair in Euglena gracilis Exposed to Ozone II. Membrane Permeability and Uptake of Metabolites

Significant injuries to the plasma membrane were detected inEuglena gracilis cells during ozone exposure (240 µ1.liter¹,delivery rate of l µmol.min^–1), as assessed by measuringthe alterations of vitamin B₁₂ and acetate uptakes and the leakageof intracellular K⁺ (Rb⁺). A rapid decrease in the uptake ofvitamin B₁₂ and acetate was observed within 15 min of treatment,indicating that both transport systems are very sensitive toO₃. On the other hand, the leakage of intracellular K⁺ ions,as measured by the efflux of ⁸⁶Rb⁺ from prelabelled cells, couldonly be detected after 30 min of O₃ exposure. These resultssuggest that the initial metabolic symptoms of injury is atthe level of the two transport systems examined and that thealteration of the membrane permeability to K⁺ ions appears asa second step in the cascade of oxidative events at the plasmamembrane level. When Euglena cells were allowed to recover underautotrophic growth conditions following O₃ treatment, vitaminB₁₂ and ⁸⁶Rb⁺ (K⁺) ions uptakes returned gradually to controllevel within 5 h of the recovery period. Acetate uptake returnedto control level at a slower rate and needed 20 h for completerecovery. These results indicate that the cells were able toactively repair most of the initial oxidative damages inducedby O₃. The metabolic significance of the repair mechanism(s)is discussed. (Received December 25, 1989; Accepted July 23, 1990)     

Growth and Partitioning in Pascopyrum smithii (C3) and Bouteloua gracilis(C4) as Influenced by Carbon Dioxide and Temperature

This study investigated how CO₂and temperature affect dry weight(d.wt) accumulation, total nonstructural carbohydrate (TNC)concentration, and partitioning of C and N among organs of twoimportant grasses of the shortgrass steppe,Pascopyrum smithiiRydb. (C₃) andBouteloua gracilis(H.B.K.) Lag. ex Steud. (C₄).Treatment combinations comprised two temperatures (20 and 35°C)at two concentrations of CO₂(380 and 750 µmol mol^-1),and two additional temperatures of 25 and 30°C at 750 µmolmol^-1CO₂. Plants were maintained under favourable nutrient andsoil moisture and harvested following 21, 35, and 49d of treatment.CO₂-induced growth enhancements were greatest at temperaturesconsidered favourable for growth of these grasses. Comparedto growth at 380 µmol mol^-1CO₂, final d.wt of CO₂-enrichedP.smithiiincreased 84% at 20°C, but only 4% at 35°C. Finald.wt ofB. graciliswas unaffected by CO₂at 20°C, but wasenhanced by 28% at 35°C. Root:shoot ratios remained relativelyconstant across CO₂levels, but increased inP. smithiiwith reductionin temperature. These partitioning results were adequately explainedby the theory of balanced root and shoot activity. Favourablegrowth temperatures led to CO₂-induced accumulations of TNCin leaves of both species, and in stems ofP. smithii, whichgenerally reflected responses of above-ground d.wt partitioningto CO₂. However, CO₂-induced decreases in plant tissue N concentrationswere more evident forP. smithii. Roots of CO₂-enrichedP. smithiihadgreater total N content at 20°C, an allocation of N below-groundthat may be an especially important adaptation for C₃plants.Tissue N contents ofB. graciliswere unaffected by CO₂. Resultssuggest CO₂enrichment may lead to reduced N requirements forgrowth in C₃plants and lower shoot N concentration, especiallyat favourable growth temperatures. Acclimation to CO₂; blue grama; Bouteloua gracilis ; carbohydrate; climate change; global change; grass; growth; growth temperature optima; nitrogen; N uptake; Pascopyrum smithii; western wheatgrass     

Gas Exchange and Carbohydrate and Nitrogen Concentrations in Leaves of Pascopyrum smithii (C3) and Bouteloua gracilis (C4) at Different Carbon Dioxide Concentrations and Temperatures

Pascopyrum smithii (C₃) andBouteloua gracilis (C₄) are importantforage grasses native to the Colorado shortgrass steppe. Thisstudy investigated photosynthetic responses of these grassesto long-term CO₂enrichment and temperature in relation to leafnonstructural carbohydrate (TNC) and [N]. Glasshouse-grown seedlingswere transferred to growth chambers and grown for 49 d at twoCO₂concentrations (380 and 750 µmol mol^-1) at 20 and 35°C, and two additional temperatures (25 and 30 °C) at750 µmol mol^-1CO₂. Leaf CO₂exchange rate (CER) was measuredat a plant's respective growth temperature and at two CO₂concentrationsof approx. 380 and 700 µmol mol^-1. Long-term CO₂enrichmentstimulated CER in both species, although the response was greaterin the C₃,P. smithii . Doubling the [CO₂] from 380 to 750 µmolmol^-1stimulated CER ofP. smithii slightly more in plants grownand measured at 30 °C compared to plants grown at 20, 25or 35 °C. CO₂-enriched plants sometimes exhibited lowerCER when compared to ambient-grown controls measured at thesame [CO₂], indicating photosynthetic acclimation to CO₂growthregime. InP. smithii , such reductions in CER were associatedwith increases in TNC and specific leaf mass, reductions inleaf [N] and, in one instance, a reduction in leaf conductancecompared to controls. InB. gracilis , photosynthetic acclimationwas observed more often, but significant changes in leaf metabolitelevels from growth at different [CO₂] were generally less evident.Temperatures considered optimal for growth (C₃: 20 °C; C₄:35 °C) sometimes led to CO₂-induced accumulations of TNCin both species, with starch accumulating in the leaves of bothspecies, and fructans accumulating only inP. smithii. Photosynthesisof both species is likely to be enhanced in future CO₂-enrichedand warmer environments, although responses will sometimes beattenuated by acclimation. Acclimation; blue grama (Bouteloua gracilis (H.B.K.) Lag ex Steud.); leaf nitrogen concentration; nonstructural carbohydrates; photosynthesis; western wheatgrass (Pascopyrum smithii (Rydb.) Love)     

Ageing of Euglena chloroplasts in vitro I. Variations in pigment pattern and in morphology

Isolated chloroplasts of Euglena gracilis Klebs were kept for10 d in complete darkness at 4 C in a maintenance buffer (pH7.5) without shaking. During incubation, the qualitative andquantitative changes in the pattern of photosynthetic pigmentswere evaluated by the combined use of spectrophotometry in thevisible range of whole chloroplasts and their acetone extracts,of in vivo spectrofluorimetry and of reversed-phase HPLC. Microscopicand submicroscopic modifications were also followed by UV andtransmission electron microscopy. The main findings were as follows: (1) a fast decay of all photosyntheticpigments, chiefly chlorophylls, not accompanied by evident signsof alteration of the thylakoid system during the first 5 d;(2) a higher stability of PSII compared to PSI and of antennacomplexes compared to the relative reaction centres during thefirst 24–48 h; (3) a low accumulation of phaeoderivativecompounds in spite of the marked decrease of chlorophyll content;(4) a lack of dephytylated compounds; (5) a quicker decay ofthe intensity of fluorescence emission with respect to the decreasingchlorophyll a content; and (6) a fast degradation of xanthophyllsand ß-carotene with the consequent lack of defencefrom the ageing oxidative stresses. This accounts for the rapidloss of pigments, although the lack of other antioxidant defencemechanisms is not excluded. The characterization of some of the steps involved in plastiddegradation may render this experimental model viable for furtherstudies on plastid senescence, a multifactorial process stillawaiting definite answers. Key words: Euglena gracilis, ageing, isolated chloroplasts, morphological changes, pigment degradation     

EUGLENA GRACILIS IN SYNCHRONOUS DIVISION I. DRY MASS AND VOLUME CHARACTERISTICS

Volume distributions and dry mass have been measured in synchronouspopulations of Euglena gracilis, grown in a salt medium at aconstant temperature. In keeping with the approximate doublingof cell number observed in each division burst, the averagedry mass and volume are doubled in each inter-division period. ¹ Present address: Department of Biology, Tokyo MetropolitanUniversity, Setagaya.ku, Tokyo. (Received February 17, 1961; )     

Pyridoxal 5-phosphate, Phenyl Phosphate and Acetyl Phosphate, as Inhibitors of Photophosphorylation Competitive with Phosphate

Pyridoxal 5-phosphate, phenyl phosphate and acetyl phosphate,as well as rß-naphthyl monophosphate, inhibited photophosphorylationof spinach chloroplasts competitively with P_i and noncompetitivelywith ADP. The apparent dissociation constant of the inhibitor-enzymecomplex (K_i) values of pyridoxal 5-phosphate, phenyl phosphateand acetyl phosphate for the P_i site were 1.1, 3.8 and 2.4 _mM,respectively. These organic phosphates inhibited Ca²⁺-ATPaseof the isolated coupling factor 1 (CF₁) (EC 3.6.1.3 [EC] ) noncompetitivelywith ATP. AMP, creatine phosphate, fructose 1,6-bisphosphate,glucose 6-phosphate, 3-phosphoglyceric acid, ribose 5-phosphateand PP_i did not significantly inhibit photophosphorylation.Like rß-naphthyl monophosphate, pyridoxal 5-phosphateand phenyl phosphate inhibited photophosphorylation and thecoupled electron transport, but were almost without effect onthe basal electron transport. On the other hand, acetyl phosphateconsiderably inhibited photophosphorylation, but had almostno effect on the coupled electron transport rate and the basalrate. The results suggest that these organic phosphates inhibitphotophosphorylation by binding at the P_i site on the activecenter of CF₁ and that their binding inhibits the ATPase activityof isolated CF₁. These four organic phosphates which inhibited photophosphorylationcompetitively with Pi could not substitute for ADP or ATP ininhibiting ferricyanide photoreduction by decreasing H⁺-permeabilitythrough CF₁ and in protecting the ATPase of isolated CF₁ againstcold-anion inactivation. ¹ This work was supported in part by Grants-in-Aid for ScientificResearch from the Ministry of Education, Science and Culture,Japan to H.S. (Received May 25, 1981; Accepted September 28, 1981)     

Adenosine 5'-Phosphosulf ate Sulfotransferase from Euglena: Enzyme-Bound Intermediates

Adenosine 5'-phosphosulfate sulfotransferase (APSST) purifiedfrom Euglena gracilis Klebs var. bacillaris mutant W₁₀BSmL byammonium sulfate precipitation, Sephadex G-100 gel filtration,reactive blue agarose, reactive dye agarose and DEAE-cellulosecan be labeled by incubation with AP³⁵S and separated from smallradioactive compounds on Sephadex G-50. Most of the label isnot exchangeable with nonradioactive APS and therefore is notassociated with bound substrate. On non-inactivating SDS-PAGE,a radioactive band at the position of native APSST tetramershows APSST activity (measured as acid-volatile radioactivity).Labeled protein hydrolyzed with Pronase yields radioactive S-sulfocysteine,indicating that at least one cysteine residue of APSST acceptsa sulfo group from APS to form E-S-SO₃^–. A labeled lowmolecular weight compound can be separated from the proteinby paper electrophoresis or by treatment with acidic proteindenaturing reagents such as trifluoroacetic acid (TFA) or trichloroaceticacid (TCA). This labeled compound (perhaps the sulfo-carrier)behaves as a strong acid on paper electrophoresis and is stabilizedby iodoacetamide or acidic conditions but degrades to thiosulfate,sulfate and other compounds as the pH is raised. The radioactivityin APSST is exchangeable with sulfite or thiosulfate. AMP inhibitsAPSST in the formation of acid-volatile radioactivity by competingwith APS, but APA inhibits APSST activity uncompetitively. AK_m of 0.1 µM for APS and K_i of 0.1 mM for AMP and 0.6mM for APA are obtained when a saturating amount of dithiothreitol(DTT) is used as the thiol. A mechanism is proposed for theinitial reaction(s) catalyzed by APSST. ¹Present address: Boyce Thompson Institute for Plant Research,Tower Road, Ithaca, NY 14853, U.S.A.