A 13C NMR study of [5,8-13C2]spermidine binding to tRNA and to Escherichia coli macromolecules.
[5,8-13C2]Spermidine was prepared by synthesis, and its binding to macromolecular structures of Escherichia coli was studied. When added to E. coli cells, the two signals of [13C]spermidine (C-5, 47.8 ppm, and C-8, 39.6 ppm; JC-C = 5.8 Hz) were strongly broadened due to binding to macromolecules. When [13C]spermidine was added to E. coli tRNA, the C-5 resonance broadened to v1/2 = 4.7 Hz, whereas the C-8 resonance broadened to v1/2 = 2.7 Hz. tRNA-bound [13C]spermidine could be chased by [12C]spermidine or spermine, but not by putrescine or cadaverine. By using mixtures of [5-13C]- and [8-13C]spermidines (where 13C-13C coupling was avoided), it was possible to estimate a dissociation constant (Kd) of 3 x 10(-3) M using the C-5 v1/2obs values and a Kd of 2.10(-3) M using the C-8 v1/2obs values. The number of spermidine-binding sites (n) could also be estimated by fitting the bound spermidine molar fraction versus tRNA concentration. Values of n = 12 +/- 2 and 14 +/- 3 were obtained for C-5 and C-8, respectively. Measurements of line narrowing at increasing Mg2+ concentrations indicated that approximately 11 spermidines (of the 12-14 bound ones) could be displaced by the former, whereas 3 spermidines remain strongly bound to the tRNA backbone. Measurements of free and bound T1 allowed the determination of a correlation time of 10(-10)s for tRNA-bound spermidine.
Frydman
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B
Facultad de Farmacia y Bioquimica, Universidad de Buenos Aires, Argentina.
de los Santos
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C
Frydman
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RB
D001665:Binding Sites; D002247:Carbon Isotopes; D004926:Escherichia coli / Q000378:metabolism*; D007553:Isotope Labeling / Q000379:methods; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008433:Mathematics; D011188:Potassium / Q000494:pharmacology; D012343:RNA, Transfer / Q000378:metabolism*; D012360:RNA, Transfer, Phe / Q000378:metabolism; D013095:Spermidine / Q000378:metabolism*; D013347:Subcellular Fractions / Q000378:metabolism
D002247:Carbon Isotopes; D012360:RNA, Transfer, Phe; D012343:RNA, Transfer; D008274:Magnesium; D011188:Potassium; D013095:Spermidine