During the course of investigation on the conformation of intact and modified tRNA molecules [1] we have observed that ethidium bromide (EB) was readily reduced by sodium borohydride, NaBH4. Similar observations have been reported by Wintermeyer and Zachau [2]. Sodium borohydride is a selective reducing agent for some atypical bases of tRNA [3] and has allowed the introduction of an intramolecular fluorescent probe into a number ofE. coli tRNA [4]. EB is an intercalating dye which has revealed to be of value in the study of nucleic acid conformation (see review in [5]). It interacts with these polymers in two ways: EB intercalates between the bases levels of standard Watson-Crick double helices with high affinity; the EB cation also binds with low affinity to the negatively charged phosphate of single stranded or double-stranded polynucleotides. We have observed that reduced ethidium Et red does not interact with nucleic acids. This reaction seems to be useful in the purification of covalently closed circular DNA. Based on PMR spectroscopy we have established the structure of the reduced form of EB. This lead: us to a reexamination of the PMR datas of Kreishman et al. for EB in aqueous solution [6]. These authors report the formation of an EB complex at high concentration in this medium. We have proposed a model for such stacked EB complex. t

,'ill f