Tertiary structure interactions of 7‐methylguanosine in yeast tRNAPhe as studied by borohydride reduction

According to the recently published crystallographic analyses of yeast tRNAPhe the positively charged m7G in the extra loop contributes to the stabilization of the tertiary structure by hydrogen bonding to the Cl 3-G22 base pair of the dihydrouridine stem [l-4] . The possibility of an additional electrostatic interaction of m7G with the closely neighbouring phosphate of A9 has been noted [4]. We have been interested in m7G as a point of specific chemical modification and cleavage of tRNAPhe [5]. Previous [6,7] and new results of NaBH4 reduction experiments prompted us to investigate the accessibility of m7G in tRNAPhe under a number of conditions. In the present study aniline catalyzed chain scission [5,8] was used as a sensitive method for the detection of m7G modification by NaBHe. The fragments Phe l-45 and Phe 47-76 are formed in this reaction, which can also be used in a preparative scale. The NaB& modification of m7G does not affect the activity of tRNAPhe in a number of biochemical assay systems. At low ionic strength m7G in tRNAPhe is reduced rather slowly as compared to its reactivity in the fragment Phe 38-76. With increasing ionic strength, however, the rate of reduction of m7G in tRNAPhe is greatly accelerated and approaches the rate observed in the case of Phe 38-76. This result is direct

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