Oxidation of 4-thiouridine in intact Escherichia coli tRNAs.

Treatment of intact tRNAs from Escherichia coli B with mild oxidizing agents, such as KI-I₂, appears to quantitatively oxidize the 4-thiouridine present in these molecules to the disulfide form as judged by the loss of absorbance near 330 nm. Chromatography of these oxidized tRNAs on Sephadex G-75 did not reveal tRNA dimers or larger aggregates, suggesting intra- rather than intermolecular disulfide-bond formation. Enzymatic hydrolyses of both unlabeled and ³⁵S-labeled oxidized tRNAs followed by chromatography on columns of Sephadex G-25 indicated that 4-thiouridine did form covalent linkages with some component(s) in the tRNA that were reversible upon reduction. It was not clear whether 4-thiouridine formed disulfides only with itself, other sulfurcontaining nucleosides, or some non-sulfur-containing component. Data presented suggest that an earlier report on the isolation of 4-thiouridylate disulfide from oxidized tRNAs of E. coli was an artifact, resulting from oxidation of the thionucleotide during chromatography on Bio-Gel.