Amino acid biosynthesis in isolated pea chloroplasts: metabolism of labeled aspartate and sulfate

Mills, W. R.; Wilson, Kenneth G.

doi:10.1016/0014-5793(78)80737-6

Cited by 23 publications

(13 citation statements)

References 18 publications

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“…This compartmentation is also in accordance with the subcellular location of the enzyme folylpolyglutamate synthetase that is involved in the synthesis of the glutamate tail of tetrahydrofolate coenzymes (17). The presence of this enzyme in plastids is of crucial importance because the chloroplastic MS, similarly to the cytosolic isoforms, cannot efficiently utilize the monoglutamate form of 5-CH 3 -H 4 PteGlu n as a methyl donor for Hcy methylation (Table II) (38,39).…”

Section: Discussionmentioning

confidence: 87%

Methionine Metabolism in Plants

Ravanel

Block

Rippert

et al. 2004

Journal of Biological Chemistry

224

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The subcellular distribution of Met and S-adenosylmethionine (AdoMet) metabolism in plant cells discloses a complex partition between the cytosol and the organelles. In the present work we show that Arabidopsis contains three functional isoforms of vitamin B 12 -independent methionine synthase (MS), the enzyme that catalyzes the methylation of homocysteine to Met with 5-methyltetrahydrofolate as methyl group donor. One MS isoform is present in chloroplasts and is most likely required to methylate homocysteine that is synthesized de novo in this compartment. Thus, chloroplasts are autonomous and are the unique site for de novo Met synthesis in plant cells. The additional MS isoforms are present in the cytosol and are most probably involved in the regeneration of Met from homocysteine produced in the course of the activated methyl cycle. Although Met synthesis can occur in chloroplasts, there is no evidence that AdoMet is synthesized anywhere but the cytosol. In accordance with this proposal, we show that AdoMet is transported into chloroplasts by a carrier-mediated facilitated diffusion process. This carrier is able to catalyze the uniport uptake of AdoMet into chloroplasts as well as the exchange between cytosolic AdoMet and chloroplastic AdoMet or S-adenosylhomocysteine. The obvious function for the carrier is to sustain methylation reactions and other AdoMet-dependent functions in chloroplasts and probably to remove S-adenosylhomocysteine generated in the stroma by methyltransferase activities. Therefore, the chloroplastic AdoMet carrier serves as a link between cytosolic and chloroplastic onecarbon metabolism.

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Section: Discussionmentioning

confidence: 87%

Methionine Metabolism in Plants

Ravanel

Block

Rippert

et al. 2004

Journal of Biological Chemistry

224

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“…3), it is possible that these two amino acids in particular became limiting during the course ofthe reaction. This was not expected, since they are on the list of those shown to be synthesized from precursors by isolated pea chloroplasts (25,26,30). The five specific polypeptides whose synthesis was stimulated at 10 min by isoleucine and threonine, before their entire effect on total incorporation was apparent, might be especially rich in these amino acids.…”

Section: Discussionmentioning

confidence: 99%

Factors Permitting Prolonged Translation by Isolated Pea Chloroplasts

Nivison¹,

Jagendorf²

1984

Plant Physiol.

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ABSTRACrThe following parameters were found to prolong the time-course of translation by isolated pea (Pisum sativum, cv Progress No. 9) chloroplasts: addition of other amino acids (an effect synergistic with sufficient free Mg"), use of lower light intensities, and additions of inorganic phosphate and ATP. In a chloroplast system which includes these parameters, active translation usually extends to almost an hour. The total amount of leucine incorporated is routinely 60 to 100 nanomoles/milligram chlorophyll and often 200 nanomoles/milligram chlorophyll. Accurate estimation of the amount of amino acid incorporated depends on supplying the labeled amino acid at a concentration sufficient to overcome isotope dilution effects from endogenous pools. Approximately 39 thylakoid and 60 stroma polypeptides were visible on autoradiographs after labeling with I3SImethionine. Label in a few of the polypeptide bands was increased or decreased by specific changes in the reaction conditions. Due to the long period of activity and the large number of labeled products, this chloroplast system should be useful for future studies of chloroplast translation.Isolated, intact chloroplasts have been used to identify products of chloroplast protein synthesis as well as to investigate posttranslational protein transport, processing, assembly, and insertion into membranes. The results of such studies may depend strongly on the characteristics and competence of the in vitro chloroplast systems. For example, in an early in vitro chloroplast system only one soluble product was detected (1), while in more recent work up to 39 thylakoid (13) and 80 soluble (10) polypeptide products have been reported. The movement across the envelope of cytoplasmically synthesized precursors to chloroplast proteins depends on an adequate supply of internal ATP (15). Processing of the chloroplast-synthesized, herbicidebinding protein (6) has been demonstrated to occur in isolated pea (9,17) and lettuce (17) chloroplasts. However, in maize the processing has been seen in vivo but so far not in organello (14) perhaps because some necessary condition(s) has not been identified. Assembly of cytoplasmically synthesized small subunits and chloroplast-synthesized large subunits of the chloroplast enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase into a holoenzyme did not occur in isolated chloroplasts until sorbitol, rather than KCI, was used as an osmoticum (2,8). Clearly, increasing the functionality of isolated chloroplasts will increase

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“…However, the enzyme has been studied from several different plant species (1,2,4,9,(19)(20)(21)(22). HSDH has been localized in the chloroplast fractions of maize (4) and pea leaves (24,25). To date, HSDH has been purified to homogeneity and well characterized from maize (18) and recently in our laboratory from carrot suspension cultures (23 …”

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confidence: 99%

Immunological Characterization of in Vitro Forms of Homoserine Dehydrogenase from Carrot Suspension Cultures

Turano¹,

Jordan²,

Matthews³

1990

Plant Physiol.

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Multiple forms of homoserine dehydrogenase (HSDH) from carrot (Daucus carota L.) have been identified. One form of HSDH (T-form) has a relative molecular weight of 240,000 and is strongly inhibited by threonine. Another form (K-form) has a relative molecular weight of 180,000 and is insensitive to inhibition by threonine. The interconversion of these two forms is dependent upon the presence or absence of threonine and potassium. Polyacrylamide electrophoretic gels stained for HSDH activity and protein, paralleled with Western blot analysis, verified the interconversion of the T-and K-forms in 5 millimolar threonine and 100 millimolar potassium, respectively. Carrot HSDH also aggregates to form higher molecular weight complexes of 240,000 up to 720,000 Mr. Polyclonal antibody from mouse was raised against the T-form (240,000 Mr) of carrot HSDH. Specificity of the mouse antisera to carrot HSDH was verified by immunoprecipitation and Western blot analysis. The T-form, K-form, and all of the higher molecular aggregates of carrot HSDH cross-reacted with the anti-HSDH antiserum. The antiserum also cross-reacted with soybean HSDH, but did not cross-react with either of the two HSDH forms found in Escherichia coi. A model for the in vivo regulation of threonine biosynthesis in the chloroplast is presented. The model is based on the interconversion of the HSDH forms by potassium and threonine.HSDH2 (EC 1.1.1.3) is a key branch-point enzyme in the biosynthetic pathway of the aspartate family of amino acids. In plants and bacteria the essential amino acids lysine, threonine, isoleucine and methionine are synthesized by this pathway (Fig. 1)

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Amino acid biosynthesis in isolated pea chloroplasts: metabolism of labeled aspartate and sulfate

Cited by 23 publications

References 18 publications

Methionine Metabolism in Plants

Methionine Metabolism in Plants

Factors Permitting Prolonged Translation by Isolated Pea Chloroplasts

Immunological Characterization of in Vitro Forms of Homoserine Dehydrogenase from Carrot Suspension Cultures

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