EFFECTS OF ORTHO-PHOSPHATE AND ADENOSINE 5'-PHOSPHATE ON THREONINE SYNTHASE AND CYSTATHIONINE GAMMA-SYNTHASE OF LEMNA-PAUCICOSTATA HEGELM 6746

被引:18
作者
GIOVANELLI, J [1 ]
MUDD, SH [1 ]
DATKO, AH [1 ]
THOMPSON, GA [1 ]
机构
[1] NIMH, GEN & COMPARAT BIOCHEM, BLDG 32, ROOM 101, 9000 ROCKVILLE PIKE, BETHESDA, MD 20892 USA
关键词
D O I
10.1104/pp.81.2.577
中图分类号
Q94 [植物学];
学科分类号
071001 ;
摘要
Inorganic phosphate (Pi) inhibits threonine synthase of Lemna, and cystathionine .gamma.-synthase less strongly. AMP is an extremely potent and structurally specific inhibitor of threonine synthase. Each inhibition progressively decreases with increasing concentrations of O-phosphohomoserine (OPH). To study the in vivo effects of these inhibitions, Lemna was grown with a range of Pi concentrations. A 25,000-fold increase in Pi concentration in the culture medium caused an increase of only 6-fold in total phosphorus of the plants. This is explained by the fact that a high affinity Pi uptake system is selectively down-regulated during growth with high concentrations of Pi. Pi and AMP in plants grown with various Pi concentrations were determined, and concentrations estimated for chloroplasts, the organelle containing threonine synthase and cystathionine .gamma.-synthase. Calculations indicated that for growth at standard external Pi (0.4 millimolar) or above, if total OPH were uniformly distributed within the plants, activities of the two enzymes in question would be severely inhibited, and each would fall two orders of magnitude below the amount required to provide threonine (plus isoleucine) or methionine adequate for growth. If OPH were restricted to chloroplasts, these inhibitions would be much less severe, resulting in enzyme activities approaching the required physiological amounts. Evidence is presented that even up to 50 millimolar external Pi, this ion does not limit production of threonine or methionine sufficiently to retard growth, consistent with the postulated localization of OPH within chloroplasts.
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页码:577 / 583
页数:7
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