The selective iodination of yeast phenylalanine transfer RNA with 125-I

A rapid method for mapping exposed cytosines in polyribonucleotides. Application to tRNATrp (yeast, beef liver)

A rapid method for mapping exposed cytosine residues in 5'-[32P]-labeled RNA molecules is suggested. The exposed cytosines (C's) are converted into uracyls (U's) by bisulphite treatment at pH 5.8 in the presence of Mg2+, followed by complete modification of the residual (non-exposed) C's by a methoxyamine and bisulphite mixture at pH 5.0. The control RNA is modified only by methoxyamine and bisulphite without the preliminary C leads to U conversion. The location of the exposed C's is determined by comparing the products of partial T1, T2, A and U2 ribonuclease digestions of the C leads to U converted and control RNAs after slab gel polyacrylamide electrophoresis and autoradiography. The method has been applied for mapping exposed cytosine bases in tRNATrp (yeast) which have been found in the anti-codon loop and at the 3'-end of the molecule. In tRNATrp (beef liver), in addition to the same exposed bases, C in the diHU-loop is exposed. The data obtained are in full agreement with what is known about exposed C's for other tRNAs.

The kinetics of bisulphite modification of reactive residues in E. coli tRNA2Phe

E coli tRNA2Phe was modified at 25 degrees C with 3M sodium bisulphite, pH6.0, for periods of up to 48 hours, Three cytadinine residues, at position 17, 74 and 75 from the 5' end were each deaminated to uridine. The 2-methylthio-N6-isopentenyl adenosine at position 37 formed a 1:1 bi-sulphite addition product which was stable to alkaii. No other residues were permanently modified. The rate of modification of each residue was first order with respect to remaining unmodified nucleotide, the time of half reaction, t1/2, being different for each residue. C17 reaction reacted at twice the rate of cytidine in PolyC, indicating that it occupied a very exposed position in the tRNA.