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Effects of Iron Deficiency on Heme Biosynthesis in Rhizobium Japonicum PAUL G

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Effects of Iron Deficiency on Heme Biosynthesis in Rhizobium Japonicum PAUL G JOURNAL OF BACTERIOLOGY, Mar. 1982, p. 1021-1026 Vol. 149, No. 3 0021-9193/82/031021-06$02.00/0 Effects of Iron Deficiency on Heme Biosynthesis in Rhizobium japonicum PAUL G. ROESSLERt* AND KENNETH D. NADLER Department of Botany and Plant Pathology, Michigan State University, East Lansing, Michigan 48824 Received 30 July 1981/Accepted 18 October 1981 The effects of iron deficiency on heme biosynthesis in Rhizobium japonicum were examined. Iron-deficient cells had a decreased maximum cell yield and a decreased cytochrome content and excreted protoporphyrin into the growth medium. The activities of the first two enzymes of heme biosynthesis, 5- aminolevulinic acid synthase (EC 2.3.1.37) and 5-aminolevulinic acid dehydrase (EC 4.2.1.24), were diminished in iron-deficient cells, but were returned to normal levels upon addition of iron to the cultures. The addition of iron salts, iron chelators, hemin, or protoporphyrin to cell-free extracts did not affect the activity of these enzymes. The addition of levulinic acid to iron-deficient cultures blocked protoporphyrin excretion and also resulted in high 5-aminolevulinic acid synthase and 5-aminolevulinic acid dehydrase activities. These results suggest the possibili- ty that rhizobial heme synthesis in the legume root nodule may be affected by the release of iron from the host plant to the bacteroids. Leghemoglobins, the hemoproteins required ing conditions (5). There have also been several for nitrogen fixation in the legume root nodule, reports suggesting that iron is required at the are unique products of the Rhizobium-legume step where coproporphyrinogen is converted to symbiosis. The heme is produced by the Rhizo- protoporphyrin (PROTO) in certain organisms bium bacteroids (8, 13, 18), whereas the apopro- (14, 22). In this investigation, we have grown tein is produced by the host plant (9, 11, 25). Rhizobium japonicum cells under iron-limiting During the latter stages of nodule formation, conditions to examine the possibility that iron leghemoglobins are produced in such large released by the host plant may play a regulatory amounts that they comprise >20% of the total role in heme biosynthesis in R. japonicum. soluble protein of the nodule, equivalent to 100 to 200 nmol of heme per g (wet weight) (18). By MATERIALS AND METHODS comparison, laboratory-cultured rhizobia con- Media and growth conditions. The chemicals used tain approximately 10 nmol of heme per g (wet for media preparation were chosen from readily avail- weight). It is thus evident that a large increase in able sources for having low amounts of iron contami- rhizobial heme synthesis occurs during nodula- nation. In addition, all culture flasks were soaked tion. However, the mechanisms involved in the overnight in 6 N HCl to remove contaminating iron. regulation of heme biosynthesis in Rhizobium Low-iron (LI) medium contained (in grams per liter have not been characterized to any great extent. of distilled, deionized water): mannitol, 2; Na2HPO4 The regulation of rhizobial heme synthesis 7H20, 1.3; NaH2PO4IH20, 0.1; MgSO4 7H20, 0.05; NaCl, 0.5; (NH4)2SO4, 1; Ca(NO3)2 4H20, 0.02; biotin might occur via the transfer of a regulatory (Sigma Chemical Co.), 0.0025; and Casamino Acids compound from the host plant to the bacteroids. (Difco Laboratories), 1. High-iron (HI) medium was Several reports have suggested that iron may obtained by adding 4 mg of ferric citrate (Baker play a regulatory role in heme biosynthesis in Chemical Co.; 16% iron by assay) per liter of LI certain organisms. The first enzyme unique to medium. Yeast extract-mannitol medium has been the heme biosynthetic pathway, 8-aminolevu- described previously (1). linic acid synthase (ALAS), has been reported to The concentration of iron in LI medium was deter- have increased activity in the presence of iron in mined with a Varian model AA6 atomic absorption animals (3, 26), plants and spectrophotometer with carbon rod attachment CRA- (16), microorganisms 90. LI medium contained 14 ng of iron per ml, corre- (23). In addition, the second enzyme of the sponding to a concentration of 0.25 p.mol of iron per pathway, 6-aminolevulinic acid dehydrase liter. HI medium contained 11.7 p.mol of iron per liter. (ALAD), has been shown to have decreased R. japonicum 31lb-110 was kindly supplied by D. C. activity in Neurospora grown under iron-limit- Weber, U.S. Department of Agriculture, Beltsville, Md. The cultures were maintained on yeast extract- t Present address: Photoconversion Branch, Solar Energy mannitol slants and were routinely tested for nodula- Research Institute, Golden, CO 80401. tion ability on soybeans (Glycine max var. Hark) as 1021 1022 ROESSLER AND NADLER J. BACTERIOL. previously described (18). Broth cultures (500 ml) for filter, A = >570 nm). Samples (5 ml) were periodically experimental assay were grown in 1-liter Erlenmeyer removed from cultures, mixed with 1 ml of concentrat- flasks at 30°C on an orbital shaker (120 rpm). Cells ed HCl, and placed in the dark at 4°C for fluorometric were routinely harvested at early stationary phase. determination after centrifugation at 30,000 x g for 15 Viable cell counts were made by serial dilution of min. culture samples into sterile 0.7% NaCl which were Rinsing the sides of the culture flask with 5 ml of 1 N then spread out on yeast extract-mannitol plates. HCl followed by centrifugation at 30,000 x g for 20 Enzyme assays. Cell-free extracts were prepared by min provided a sample suitable for spectrophotometric sonication as previously described (1), and results are analysis of the excreted porphyrin in acidic solution. presented as the average value of two or more experi- Absorption spectra were obtained with a Cary 15 ments, with the range of variation indicated. recording spectrophotometer. (i) ALAS. 8-Aminolevulinic acid production in cell- The methyl ester of the excreted porphyrin was free extracts was measured by a modification of the obtained by rinsing the sides of the culture flask with a method of Mauzerall and Granick (17). The reaction methanol-sulfuric acid solution (20:1, vol/vol). After mixture contained (in millimoles per liter): HEPES (N- this solution was allowed to remain in the dark over- x-hydroxyethylpiperazine-N'-2-ethane-sulfonic acid) night, the porphyrin methyl ester was purified by the buffer (pH 8.0), 100; MgCI2, 16.5; Na2-succinate, 100; method of Doss and Ulshofer (12). The absorption glycine, 100; ATP, 7; coenzyme A, 0.05; pyridoxal spectrum of the porphyrin methyl ester at neutral pH phosphate, 0.3; 2-mercaptoethanol, 0.5; and the cell- could then be obtained. Additional analysis of the free extract to give a final protein concentration of excreted porphyrin along with porphyrin methyl ester between 0.5 and 1.0 mg/ml. After incubation for 2 h at standards (obtained from Sigma Chemical Co.) was 30°C, the reaction was stopped by placing the tubes on accomplished by thin-layer chromatography in the ice and adding trichloroacetic acid to a final concentra- three different systems described below. tion of 5%. After the precipitated protein was removed (iii) Protein. The concentration of protein in extracts by centrifugation, 1 ml of the supernatant solution was was measured by the method of Bradford (2), using combined with 0.25 ml of 0.65 M Na3PO4 and 0.1 ml of bovine gamma -globulin as a standard. ethylacetoacetate and placed in a boiling-water bath for 15 min. After cooling, 1.35 ml of modified Ehrlich RESULTS reagent (24) was added to each tube, and the resulting color complex was analyzed after 15 min in a spectro- Effects of iron deficiency on growth rate. The photometer (Gilson model 240) at a wavelength of 553 initial growth rates of cells in low- and high-iron nm. The molar extinction coefficient of the 6-aminole- media were identical, but after five generations vulinic acid pyrrole-Ehrlich complex at 553 nm was the growth rate of cells in LI medium rapidly determined to be 6.2 x 104. A unit of ALAS activity is decreased (Fig. 1), leading to a short stationary defined as 1 nmol of 6-aminolevulinic acid formed per h. (ii) ALAD. Porphobilinogen production in cell-frec extracts was measured colorimetrically, using modi- fied Ehrlich reagent as previously described (18) ex- cept that HEPES buffer (pH 7.5) was used in place of Tris-hydrochloride buffer. The molar extinction coeffi- cient of the porphobilinogen-Ehrlich complex at 555 nm was determined to be 5.0 x 104. A unit of ALAD activity is defined as 1 nmol of porphobilinogen formed per h. E C (iii) Succinic thiokinase. Succinic thiokinase activity U 0ffi was estimated by the detection of succinic hydroxa- S C. mate generated from succinyl-coenzyme A as previ- ._ E ously described (1). A unit of succinic thiokinase D activity is defined as 1 ,umol of succinyl-coenzyme A 10 formed per h. Analytical methods. (i) Cytochromes. Difference spectra (sodium dithionite-reduced minus ferricya- 6 *> nide-oxidized) were used to detect cytochromes in cell-free extracts obtained by sonicating cells in 0.1 M 44< sodium phosphate buffer (pH 7.0). Extracts were 24 clarified by centrifugation at 30,000 x g for 15 min, I4o-i and the protein concentration was adjusted to 3.0 mg/ ml. Difference spectra were recorded with a Cary 15 Hours recording spectrophotometer. FIG. 1. ALAS activity and growth of R.japonicuin (ii) Porphyrins. Porphyrins excreted by R. japoni- in HI and LI media. ALAS activity was measured as cum cells were insoluble in the culture medium and described in the text. Closed symbols refer to cell adhered either to the sides of the culture flask or to the counts; open symbols refer to ALAS activity.
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