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1.
Potential of a 7-dimethylallyltryptophan synthase as a tool for production of prenylated indole derivatives 总被引:2,自引:0,他引:2
Recently, a gene for a 7-dimethylallyltryptophan synthase (7-DMATS) was identified in Aspergillus fumigatus and its enzymatic function was proven biochemically. In this study, the behaviour of 7-DMATS towards aromatic substrates was investigated and compared with that of the 4-dimethylallyltryptophan synthase FgaPT2 from the same fungus. In total, 24 simple indole derivatives were tested as potential substrates for 7-DMATS. With an exception of 7-methyltryptophan, all of the substances were accepted by 7-DMATS and converted to their prenylated derivatives, indicating a more flexible substrate specificity of 7-DMATS in comparison to that of FgaPT2. The relative activities of 7-DMATS towards these substrates were from 4% to 89% of that of L-tryptophan, much higher than that of FgaPT2. Structural elucidation of the isolated enzymatic products by nuclear magnetic resonance and mass spectrometry analysis proved unequivocally the prenylation at position C7 of the indole ring. Overnight incubation with eight substances showed that the conversion ratios were in the range of 55.9% to 99.7%. This study provided an additional example that prenylated indole derivatives can be effectively produced by using the overproduced and purified 7-DMATS. 相似文献
2.
Two dimethylallyltryptophan synthases, FgaPT2 and 7-DMATS, which catalysed the prenylation of l-tryptophan at positions C4 and C7, respectively, have been recently identified in Aspergillus fumigatus and proven biochemically. These enzymes were successfully used for the production of monoprenylated indole derivatives. In
this study, we showed that C4,C7-diprenylated indole derivatives, e.g. 4,7-di-(dimethylallyl)-l-tryptophan, 4,7-di-(dimethylallyl)-l-abrine and 4,7-di-(dimethylallyl)-11-methyltryptophan, could be conveniently produced by tandem incubation of both enzymes.
The structures of the isolated enzymatic products were elucidated by NMR and MS analyses. High conversion yields of up to
93% were achieved by an incubation sequence of FgaPT2 followed by 7-DMATS. The results reported in this study demonstrated
the potential of secondary metabolite enzymes as promising tools for the production of designed compounds. 相似文献
3.
The genome sequencing of Streptomyces coelicolor A3(2) has lead to the identification of numerous cryptic gene clusters involved in the biosynthesis of secondary metabolites; throwing open the challenge of identifying the enzymatic functions that the gene clusters are associated with. In this work, we report the biochemical characterization of one such cryptic gene, SCO7467 from S. coelicolor A3(2), which is annotated as a prenyltransferase. Based on LC–MS and 2D-NMR studies, we show that SCO7467 acts as a 5-dimethylallyl tryptophan synthase (5-DMATS), and catalyzes the transfer of a dimethylallyl group to the C-5 position of the indole ring of l-tryptophan. The studies indicate that SCO7467 could be involved in the synthesis of C-5 prenylated indole alkaloids, which may exhibit unique pharmacological and biological properties. 相似文献
4.
Siu-Leung Lee Heinz G. Floss Peter Heinstein 《Archives of biochemistry and biophysics》1976,177(1):84-94
Dimethylallylpyrophosphate:l-tryptophan dimethylallyltransferase (DMAT synthetase), the first pathway-specific enzyme of ergot alkaloid biosynthesis, has been isolated from mycelia of Claviceps sp., strain SD 58, and purified to apparent homogeneity. The enzyme reaction products were identified as l-4-(γ,γ-dimethylallyl)tryptophan and inorganic pyrophosphate. DMAT synthetase is a single subunit protein of molecular weight 70,000–73,000 and has an isoelectric point at pH 5.8. The enzyme is activated by Fe2+, Mg2+, and particularly Ca2+; Km values for l-tryptophan and dimethylallylpyrophosphate were determined to be 0.067 and 0.2 mm, respectively. Kinetic analysis indicated that the DMAT synthetase reaction proceeds by a sequential rather than a ping-pong mechanism. 相似文献
5.
《Enzyme and microbial technology》1987,9(12):721-725
An enzymatic production of l-tryptophan from dl-5-indolylmethylhydantoin by the action of hydantoinase and carbamoylase has been investigated. A strain identified as (Flavobacterium) sp. I-3 isolated from soil was found to form l-tryptophan from dl-5-indolylmethylhydantoin. Cultural conditions for the formation of the l-tryptophan-forming activity were investigated, and the highest activity of 0.81 μmol min−1of l-tryptophan formed per 1 ml of culture broth (hydantoinase, 3.6 μmol min−1of N-carbamoyl-l-tryptophan formed per 1 ml of culture broth; carbamoylase, 0.92 μmol min−1of l-tryptophan formed per 1 ml of culture broth) was obtained. These activities were found to be inducible and intracellular. Optimization of the parameters of the conversion reaction resulted in accumulation of 50 mg of l-tryptophan per 1 ml of cultural broth per day. The conversion yield from dl-5-indolylmethylhydantoin was about 100%. Accumulated l-tryptophan was readily isolated in pure form by ordinary procedures. 相似文献
6.
Suqin Yin Xia Yu Qing Wang Xiao-Qing Liu Shu-Ming Li 《Applied microbiology and biotechnology》2013,97(4):1649-1660
A putative brevianamide F reverse prenyltransferase gene brePT was amplified from Aspergillus versicolor NRRL573 by using primers deduced from its orthologue notF in Aspergillus sp. MF297-2 and overexpressed in Escherichia coli. The soluble His-tagged protein BrePT was purified to near homogeneity and assayed with tryptophan-containing cyclic dipeptides in the presence of dimethylallyl diphosphate. BrePT showed much higher flexibility towards its aromatic substrates than NotF and accepted all of the 14 tested tryptophan-containing cyclic dipeptides. Structure elucidation of the enzyme products by NMR and MS analyses proved unequivocally the highly regiospecific reverse prenylation at C2 of the indole nucleus. K M values of BrePT were determined for its putative substrates brevianamide F and DMAPP at 32 and 98 μM, respectively. Average turnover number (k cat) at 0.4 s?1 was calculated from kinetic data of brevianamide F and DMAPP. K M values in the range of 0.082–2.9 mM and k cat values from 0.003 to 0.15 s?1 were determined for other 11 cyclic dipeptides. Similar to known fungal indole prenyltransferases, BrePT did not accept geranyl or farnesyl diphosphate as prenyl donor for its prenylation. 相似文献
7.
A functional map of the replicator region of the octopine Ti plasmid 总被引:14,自引:0,他引:14
A hybrid plasmid of pUB 110 (Neor) and pAB 124 (Tcr) has been constructed and shown to have a NeosTcr phenotype in Bacillus subtilis. A derivative of this pUB 110:pAB 124 hybrid has been isolated, pAB 324, which has the expected NeorTcr phenotype. A restriction endonuclease cleavage map of pAB 324 was compared to that of the parent hybrid. This showed that pAB 324 contained a minimum of two deletions and one insertion. This insertion (approximately 1.0 Md) has been identified as originating from the Bacillus subtilis chromosome. 相似文献
8.
Elena Ostertag Liujuan Zheng Karina Broger Thilo Stehle Shu-Ming Li Georg Zocher 《Journal of molecular biology》2021,433(2):166726
Prenylation is a process widely prevalent in primary and secondary metabolism, contributing to functionality and chemical diversity in natural systems. Due to their high regio- and chemoselectivities, prenyltransferases are also valuable tools for creation of new compounds by chemoenzymatic synthesis and synthetic biology. Over the last ten years, biochemical and structural investigations shed light on the mechanism and key residues that control the catalytic process, but to date crucial information on how certain prenyltransferases control regioselectivity and chemoselectivity is still lacking. Here, we advance a general understanding of the enzyme family by contributing the first structure of a tryptophan C5-prenyltransferase 5-DMATS. Additinally, the structure of a bacterial tryptophan C6-prenyltransferase 6-DMATS was solved. Analysis and comparison of both substrate-bound complexes led to the identification of key residues for catalysis. Next, site-directed mutagenesis was successfully implemented to not only modify the prenyl donor specificity but also to redirect the prenylation, thereby switching the regioselectivity of 6-DMATS to that of 5-DMATS. The general strategy of structure-guided protein engineering should be applicable to other related prenyltransferases, thus enabling the production of novel prenylated compounds. 相似文献
9.
A 60 day feeding trial was conducted to study the effect of dietary l-tryptophan on thermal tolerance and oxygen consumption rate of freshwater fish, mrigala, Cirrhinus mrigala reared under ambient temperature at low and high stocking density. Four hundred eighty fingerlings were distributed into eight experimental groups. Four groups each of low density group (10 fishes/75 L water) and higher density group (30 fishes/75 L water) were fed a diet containing 0, 0.68, 1.36 or 2.72% l-tryptophan in the diet, thus forming eight experimental groups namely, Low density control (LC) (basal feed +0% l-tryptophan); LT1 (basal feed+0.68% l-tryptophan); LT2 (basal feed+1.36% l-tryptophan); LT3 (basal feed+2.72% l-tryptophan); high density control (HC) (basal feed+0% l-tryptophan); HT1 (basal feed+0.68% l-tryptophan); HT2 (basal feed+1.36% l-tryptophan); and HT3 (basal feed+2.72% l-tryptophan) were fed at 3% of the body weight. The test diets having crude protein 34.33±0.23 to 35.81±0.18% and lipid 423.49±1.76 to 425.85±0.31 K Cal/100 g were prepared using purified ingredients. The possible role of dietary l-tryptophan on thermal tolerance and oxygen consumption rate was assessed in terms of critical thermal maxima (CTMax), critical thermal minima (CTMin), lethal thermal maxima (LTMax) and lethal thermal minima (LTMin). The CTMax, CTMin, LTMax and LTMin values were found to be significantly higher (p<0.05) in the treatment groups with CTMax 42.94±0.037 (LT2); LT Max 43.18±0.070 (LT2); CTMin 10.47±0.088 (LT2) and LTMin 9.42±0.062 (LT3), whereas the control group showed a lower tolerance level. The same trend was observed in the high density group (CTMax 42.09±0.066 (LT3); LTMax 43 23±0.067 (HT3); CTMin 10.98±0.040 (HT3) and LTMin 9.74±0.037 (HT3). However, gradual supplementation of dietary l-tryptophan in the diet significantly reduced the oxygen consumption rate in both the low density group (Y=−26.74x+222.4, r²=0.915) and the high density group (Y=−32.96x+296.5, r²=0.8923). Dietary supplementation of l-tryptophan at a level of 1.36% improved the thermal tolerance level and reduced the oxygen consumption rate in C. mrigala fingerlings. 相似文献
10.
Aili Fan ) Georg Zocher Edyta Stec Thilo Stehle Shu-Ming Li ) 《The Journal of biological chemistry》2015,290(3):1364-1373
The tryptophan prenyltransferases FgaPT2 and 7-DMATS (7-dimethylallyl tryptophan synthase) from Aspergillus fumigatus catalyze C4- and C7-prenylation of the indole ring, respectively. 7-DMATS was found to accept l-tyrosine as substrate as well and converted it to an O-prenylated derivative. An acceptance of l-tyrosine by FgaPT2 was also observed in this study. Interestingly, isolation and structure elucidation revealed the identification of a C3-prenylated l-tyrosine as enzyme product. Molecular modeling and site-directed mutagenesis led to creation of a mutant FgaPT2_K174F, which showed much higher specificity toward l-tyrosine than l-tryptophan. Its catalytic efficiency toward l-tyrosine was found to be 4.9-fold in comparison with that of non-mutated FgaPT2, whereas the activity toward l-tryptophan was less than 0.4% of that of the wild-type. To the best of our knowledge, this is the first report on an enzymatic C-prenylation of l-tyrosine as free amino acid and altering the substrate preference of a prenyltransferase by mutagenesis. 相似文献
11.
M.T. Travers 《生物化学与生物物理学报:生物膜》2004,1661(1):106-112
The activity and expression of indoleamine 2,3-dioxygenase together with l-tryptophan transport has been examined in cultured human breast cancer cells. MDA-MB-231 but not MCF-7 cells expressed mRNA for indoleamine 2,3-dioxygenase. Kynurenine production by MDA-MB-231 cells, which was taken as a measure of enzyme activity, was markedly stimulated by interferon-γ (1000 units/ml). Accordingly, l-tryptophan utilization by MDA-MB-231 cells was enhanced by interferon-γ. 1-Methyl-dl-tryptophan (1 mM) inhibited interferon-γ induced kynurenine production by MBA-MB-231 cells. Kynurenine production by MCF-7 cells remained at basal levels when cultured in the presence of interferon-γ. l-Tryptophan transport into MDA-MB-231 cells was via a Na+-independent, BCH-sensitive pathway. It appears that system L (LAT1/CD98) may be the only pathway for l-tryptophan transport into these cells. 1-Methyl-d,l-tryptophan trans-stimulated l-tryptophan efflux from MDA-MB-231 cells and thus appears to be a transported substrate of system L. The results suggest that system L plays an important role in providing indoleamine-2,3-dioxygenase with its main substrate, l-tryptophan, and suggest a mechanism by which estrogen receptor-negative breast cancer cells may evade the attention of the immune system. 相似文献
12.
Recently, the prenyltransferase SirD was found to be responsible for the O-prenylation of tyrosine in the biosynthesis of sirodesmin PL in Leptosphaeria maculans. In this study, the behavior of SirD towards phenylalanine/tyrosine and tryptophan derivatives was investigated. Product
formation has been observed with 12 of 19 phenylalanine/tyrosine derivatives. It was shown that the alanine structure attached
to the benzene ring and an electron donor, e.g., OH or NH2, at its para-position are essential for the enzyme activity. Modifications were possible both at the side chain and the benzene ring.
Enzyme products from seven phenylalanine/tyrosine derivatives were isolated and characterized by MS and NMR analyses including
HSQC and HMBC and proven to be O- or N-prenylated derivatives at position C4 of the benzene rings. K
M
values of six selected derivatives were found in the range of 0.10–0.68 mM. Catalytic efficiencies (K
cat/K
M
) were determined in the range of 430–1,110 s−1·M−1 with l-tyrosine as the best substrate. In addition, 7 of 14 tested tryptophan analogs were also accepted by SirD and converted to
C7-prenylated derivatives, which was confirmed by comparison with products obtained from enzyme assays using a 7-dimethylallyltryptophan
synthase 7-DMATS from Aspergillus fumigatus. 相似文献
13.
14.
Chemically oxidized, catalytically inactive, pseudomonad l-tryptophan-2,3-dioxygenase (EC 1.13.1.12) can be photoactivated aerobically as well as anaerobically by light of wavelength less than 360 nm. The substrate, l-tryptophan, must be present for photoactivation to proceed. In these studies, a CCl4 filter was used to block light of wavelength less than 265 nm, preventing photolysis of water and the concomitant production of H2O2 (known reductant of tryptophan oxygenase). Photoactivation is not inhibited by superoxide dismutase or formate and is only slightly inhibited by catalase. Nonsubstrate analogues of l-tryptophan, 5-fluorotryptophan (binds to the catalytic site), and α-methyltryptophan (binds to the allosteric site), separately or in concert, do not mediate photoactivation, while another substrate, 6-fluorotryptophan, can. Saturation of the allosteric site with α-methyltryptophan increases the extent of photoactivation in the presence of a nonsaturating level of l-tryptophan, indicating that photoactivation is dependent on the extent of saturation of the catalytic site by l-tryptophan. During the time course of photoactivation, catalytic activity increases faster than does the formation of ferroheme enzyme, indicating that the fully reduced enzyme, (ferroheme)2(Cu+)2, is formed from the fully oxidized enzyme, (ferriheme)2(Cu2+)2, subsequent to photoactivation. A significant amount of the half-reduced, catalytically active enzyme, (ferriheme)2(Cu+)2, exists during the time course of photoactivation. We propose that the mechanism by which electrons enter tryptophan oxygenase is via “electron ejection” [T. R. Hopkins and R. Lumry (1972) Photochem. Photobiol.15, 555–566] from a photoexcited l-tryptophan bound at, the catalvtic site. 相似文献
15.
A bacterium, tentatively identified as an Arthrobacter sp., was isolated from flooded soil that was incubated at 35°C and repeatedly treated with carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl N-methylcarbamate). This bacterium exhibited an exceptional capacity to completely mineralize the ring-labeled 14C in carbofuran to 14CO2 within 72 to 120 h in a mineral salts medium as a sole source of carbon and nitrogen under aerobic conditions. Mineralization was more rapid at 35°C than at 20°C. No degradation of carbofuran occurred even after prolonged incubation under anaerobic conditions. The predicted metabolites of carbofuran, 7-phenol (2,3-dihydro-2,2-dimethyl-7-benzofuranol) and 3-hydroxycarbofuran, were also metabolized rapidly. 7-Phenol, although formed during carbofuran degradation, never accumulated in large amounts, evidently because of its further metabolism through ring cleavage. The bacterium readily hydrolyzed carbaryl (1-naphthyl N-methylcarbamate), but its hydrolysis product, 1-naphthol, resisted further degradation by this bacterium. 相似文献
16.
Qingmei Liu Muhammad Zubair Siddiqi Mi-Sun Kim Sang Yong Kim Wan-Taek Im 《Journal of microbiology (Seoul, Korea)》2017,55(7):525-530
A Gram-negative, non-motile, aerobic, and rod-shaped bacterial strain designated as BR5-28T was isolated from the soil of a ginseng field at Baekdu Mountain Korea, and its taxonomic position was investigated using a polyphasic approach. Strain BR5-28T grew at 10–42°C (optimum temperature, 30°C) and pH 5.5–8.5 (optimum pH, 7.0) on R2A agar medium without additional NaCl supplementation. Strain BR5- 28T exhibited β-glucosidase activity, which was responsible for its ability to transform the ginsenosides Rb1 and Rd (the two dominant active components of ginseng) to compound-K. Based on 16S rRNA gene phylogeny, the novel strain showed a new branch within the genus Mucilaginibacter of the family Sphingobacteriaceae, and formed clusters with Mucilaginibacter frigoritolerans FT22T (95.8%) and Mucilaginibacter gotjawali SA3-7T (95.7%). The G+C content of the genomic DNA was 45.1%. The predominant respiratory quinone was MK-7 and the major fatty acids were summed feature 3 (comprising C16:1 ω6c and/or C16:1 ω7c), iso-C15:0 and anteiso-C15:0. The major polar lipids were diphosphatidylglycerol, phosphatidylglycerol and phosphatidylethanolamine. Strain BR5-28T was differentiated genotypically and phenotypically from the recognized species of the genus Mucilaginibacter. The isolate therefore represents a novel species, for which the name Mucilaginibacter hankyongensis sp. nov. is proposed, with the type strain BR5-28T (=KCTC 22274T =DSM 21151T). 相似文献
17.
Shuhuai Yu Xiao Wang Tao Zhang Timo Stressler Lutz Fischer Bo Jiang Wanmeng Mu 《PloS one》2015,10(11)
Previously, a di-D-fructofuranose 1,2’:2,3’ dianhydride (DFA III)-producing strain, Arthrobacter aurescens SK8.001, was isolated from soil, and the gene cloning and characterization of the DFA III-forming enzyme was studied. In this study, a DFA III hydrolysis enzyme (DFA IIIase)-encoding gene was obtained from the same strain, and the DFA IIIase gene was cloned and expressed in Escherichia coli. The SDS-PAGE and gel filtration results indicated that the purified enzyme was a homotrimer holoenzyme of 145 kDa composed of subunits of 49 kDa. The enzyme displayed the highest catalytic activity for DFA III at pH 5.5 and 55°C, with specific activity of 232 U mg-1. K
m and V
max for DFA III were 30.7 ± 4.3 mM and 1.2 ± 0.1 mM min-1, respectively. Interestingly, DFA III-forming enzymes and DFA IIIases are highly homologous in amino acid sequence. The molecular modeling and docking of DFA IIIase were first studied, using DFA III-forming enzyme from Bacillus sp. snu-7 as a template. It was suggested that A. aurescens DFA IIIase shared a similar three-dimensional structure with the reported DFA III-forming enzyme from Bacillus sp. snu-7. Furthermore, their catalytic sites may occupy the same position on the proteins. Based on molecular docking analysis and site-directed mutagenesis, it was shown that D207 and E218 were two potential critical residues for the catalysis of A. aurescens DFA IIIase. 相似文献
18.
Hyaluronidases are a family of enzymes that degrade hyaluronic acid (hyaluronan, HA) and widely used in many fields. A hyaluronidase producing bacteria strain was screened from the air. 16S ribosomal DNA (16S rDNA) analysis indicated that the strain belonged to the genus Bacillus, and the strain was named as Bacillus sp. A50. This is the first report of a hyaluronidase from Bacillus, which yields unsaturated oligosaccharides as product like other microbial hyaluronate lyases. Under optimized conditions, the yield of hyaluronidase from Bacillus sp. A50 could reach up to 1.5×104 U/mL, suggesting that strain A50 is a good producer of hyaluronidase. The hyaluronidase (HAase-B) was isolated and purified from the bacterial culture, with a specific activity of 1.02×106 U/mg protein and a yield of 25.38%. The optimal temperature and pH of HAase-B were 44°C and pH 6.5, respectively. It was stable at pH 5–6 and at a temperature lower than 45°C. The enzymatic activity could be enhanced by Ca2+, Mg2+, or Ni2+, and inhibited by Zn2+, Cu2+, EDTA, ethylene glycol tetraacetic acid (EGTA), deferoxamine mesylate salt (DFO), triton X-100, Tween 80, or SDS at different levels. Kinetic measurements of HAase-B towards HA gave a Michaelis constant (K
m) of 0.02 mg/mL, and a maximum velocity (V
max) of 0.27 A
232/min. HAase-B also showed activity towards chondroitin sulfate A (CSA) with the kinetic parameters, K
m and V
max, 12.30 mg/mL and 0.20 A
232/min respectively. Meanwhile, according to the sequences of genomic DNA and HAase-B’s part peptides, a 3,324-bp gene encoding HAase-B was obtained. 相似文献
19.
Effects of temperature and irradiance on quantum yield of PSII photochemistry and xanthophyll cycle in a tropical and a temperate species 总被引:1,自引:0,他引:1
The effect of a wide range of temperatures (?15 and 60°C) in darkness or under strong irradiation [1,600 μmol(photon) m?2 s?1] on quantum yield of photosystem II photochemistry and xanthophyll cycle pigments was investigated in a tropical fruit crop (Musa sp.) and a temperate spring flowering plant (Allium ursinum L.). In darkness within the nonlethal thermal window of A. ursinum (from ?6.7 to 47.7°C; 54.5 K) and of Musa sp. (from ?2.2°C to 49.5°C; 51.7 K) maximal quantum yield of PSII photochemistry (Fv/Fm) was fairly unaffected by temperature over more than 40 K. At low temperature Fv/Fm started to drop with ice nucleation but significantly only with initial frost injuries (temperature at 10% frost damage; LT10). The critical high temperature threshold for PSII (Tc) was 43.8°C in A. ursinum and 44.7°C in Musa sp. Under strong irradiation, exposure to temperatures exceeding the growth ones but being still nonlethal caused photoinhibition in both species. Severity of photoinhibition increased with increasing distance to the growth temperature range. ΔF/Fm′ revealed distinctly different optimum temperature ranges: 27–36°C for Musa sp. and 18–27°C for A. ursinum exceeding maximum growth temperature by 2–7 K. In both species only at temperatures > 30°C zeaxanthin increased and violaxanthin decreased significantly. At nonlethal low temperature relative amounts of xanthophylls remained unchanged. At temperatures > 40°C β-carotene increased significantly in both species. In Musa sp. lutein and neoxanthin were significantly increased at 45°C, in A. ursinum lutein remained unchanged, neoxanthin levels decreased in the supraoptimal temperature range. In darkness, Fv/Fm was highly temperature-insensitive in both species. Under strong irradiation, whenever growth temperature was exceeded, photoinhibition occurred with xanthophylls being changed only under supraoptimal temperature conditions as an antiradical defence mechanism. 相似文献
20.
Incorporation of C-photosynthate into major chemical fractions of source and sink leaves of cottonwood
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The incorporation and distribution of photosynthetically fixed 14CO2 was followed for 48 hours in a recently matured source leaf (LPI 7) and in young expanding source and sink leaves (LPI 4) of cottonwood (Populus deltoides Bartr.). The major chemical constituents of leaf laminae and petioles were separated by sequential solvent extractions and enzyme hydrolyses. Two hours after labeling, about 80% of the 14C was found in water-alcohol-soluble constituents in the mature source lamina as compared to about 45% in those of the young expanding leaf. In both mature and expanding source leaves the water-alcohol-soluble constituents decreased while the CHCl3-soluble and -insoluble compounds increased with time. After 48 hours, 7 and 37% of the total 14C was recovered from structural carbohydrates and from protein + CHCl3-soluble fractions, respectively, in the mature source leaf; and 4 and 65%, respectively, in the young source leaf. When the distribution of 14C among major chemical fractions was calculated on per cent dpm/mg basis, the data showed that a young sink leaf incorporated over twice as much 14C into structural carbohydrates as a young source leaf (11% versus 4%). However, when calculated on an absolute dpm/mg basis, activity in this fraction of the young source leaf exceeded that in the sink leaf by a ratio of about 11:1 (9528 versus 845 dpm/mg). Thus, most of the material for synthesis of structural carbohydrates was derived from in situ photosynthate. 相似文献