Ionic strength dependent conformational changes of t-RNA studied by circular polarization of phosphorescence

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Abstract
The circularly polarized phosphorescence emitted by the 4-thiouridine base was measured for bulk Escherichia coli tRNA, E. coli tRNA1Val, E. coli tRNAfMet and E. coli tRNAPhe as a function of ionic strength in the presence of 1 mM Mg2+. The emission anisotropy factor, gem, is dependent on the degree of local stacking in the vicinity of the chromophore. For bulk tRNA and tRNAfMet a marked similarity was observed between the behavior of gem and the translation diffusion coefficient .**GRAPHIC**. This was interpreted to mean that a relationship exists between the local stacking around the 4-thiouridine chromophore and the overall hydrodynamic shape of the tRNA molecule. Although a general similarity of behavior was observed, each species of tRNA examined exhibited a distinctive dependence of conformation on salt concentration. tRNAPhe is a particular exception in the low-salt region (< 50 mM). The value of the phosphorescence anisotropy factor is remarkably larger (.apprx. 50-fold) than the value observed for the absorption anisotropy factor. Minor conformational changes in the tRNA molecule are more readily detected by circular polarization of phosphorescence than by circular dichroism.

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