Fluorescence study of S4 and S7 ribosomal proteins complexed to 16 S RNA

Fluorescence studies on native and bound to trifluraline soy bean Lb"a" in the enhanced N2 fixation

The differences in the tryptophan (Trp) fluorescence of native (control) Lb"a" and experimental substance isolated from nodules of the Williams' soy beans variety treated with trifluraline at a concentration of 2.1 x 10(-10) M have been studied. A positively charged environment has been proved for the tryptophans of the native Lb"a" and a negative one for the tryptophans of the experimental Lb"a". The difference in the tryptophan emission spectra at lambdaex = 280 and 300 nm may be assigned to conformational alterations occurring in the experimental Lb"a". This is also confirmed by the greater energy transfer from tyrosine to tryptophan in the experimental Lb"a"--30% compared to the 10% in the native Lb"a". The value of the constant of acrylamide quenching (Ksv = 2.77 M(-1)) shows that the tryptophans are buried more deeply in the experimental Lb"a" than in the native Lb"a" (Ksv = 4 M(-1)). They are substantially lower than Ksv of the standard compound N-Ac-Trp-NH2 (16.30 M(-1)). The activation energy (Ea) of the thermal quenching of tryptophan fluorescence is higher for the experimental Lb"a" (37 kJ mol(-1)) as compared to the standard compound N-Ac-Trp-NH2 (24 kJ mol(-1)) and the native Lb "a" (32 kJ mol(-1)). The dissociation constant of the complex of trifluraline with Lb "a" (6.32 x 10(-11) M) has been determined as well as the stoichiometric ratio trifluraline/Lb"a" (1:1). The estimated nitrogenase activity (microM/gfrw h) and the total Lb (mg/gfrw) for trifluraline are higher as compared to those for the control.

Conformational changes of 30S ribosomes measured by intrinsic and extrinsic fluorescence

The intrinsic tryptophan fluorescence and the fluorescence of N-(3-pyrene)maleimide, a covalently bound sulfhydryl-specific extrinsic probe, have been used to study the conformation of the 30S ribosomal subunit of Escherichia coli. (a) The tryptophan fluorescenct spectrum of the free ribosomal proteins is shifted to shorter wavelengths than that of free tryptophan. When the proteins are incorporated into the organized structure of the ribosome, there is a small additional blue shift and the emission band becomes narrower. In 6 M urea, the spectrum of the proteins, whether free or in the ribosome, becomes identical with that of the amino acid, reflecting exposure of previously shielded tryptophan residues. (b) When magnesium-depleted ribosomes are unfolded at low ionic strength, the tryptophan fluorescence spectrum changes, although circular dichroism shows no change in alpha-helix content of the proteins. (c) Intrinsic and extrinsic fluorescence were both found to be sensitive to a limited and fully reversible transition that takes place when ribosomes are incubated under conditions that increase their activity in vitro. This suggests that both probes may be of use in monitoring conformational changes that occur under conditions consistent with activity. The kinetics of the concurrent changes in extrinsic fluorescence and aminoacyl-tRNA binding activity were compared. (d) Conditions are described for labeling ribosomes with N-(3-pyrene)maleimide without impairing their activity.