Iodination-deiodination. A radiochemical method for detection of structure and changes in structure in RNA.

Bound iodine is released from radioiodinated nucleotides in polymers exposed to sodium bisulfite. The rate of bisulfite-catalyzed deiodination of pyrimidines can be controlled both by change of temperature of pH and is also dependent on the molecular association of the nucleotide. The rate of release of iodine from iodocytidine in polycytidylate is greater than the rate of elimination from RNA. Experiments testing the influence of base-pairing of the iodopyrimidines in synthetic polynucleotides showed that pairing of the substituted nucleotide protected the iodine bond. The rates of bisulfite-catalyzed deiodination of several radioiodinated RNAs were measured. The action of bisulfite on all single stranded RNAs tested was multiphasic consisting of a rapid early deiodination reaction supplanted by a slower phase which was followed by reacceleration of release. The release of iodine from double stranded RNA and DNA-RNA duplexes was retarded in comparison with the release from ribosomal and messenger RNA fractions. The deiodination profiles of single and double stranded RNA suggested that the intermediate stage iodine release is governed by melting of paired zones of low stability. Late release may result from destablization of the molecule through the addition of bisulfite to the pyrimidine ring or deamination. The effect of several substances expected to complex with polynucleotides was tested. Acridine orange and ethidium bromide increased loss of iodine from ribosomal RNA but slightly decreased elimination from double stranded viral RNA. A basic protein fraction isolated from ribosomal particles accelerated the deiodination of ribosomal RNA. While the destabilization caused by this protein fraction was greater than that caused by an equal amount of albumin, as tested the effect was non-specific. The results show that a change in sensitivity to chemical deiodination may folow the interaction of small amounts of protein with polynucleotides.