Platinum binds selectively to phosphorothioate groups in mono- and polynucleotides: a general method for heavy metal staining of specific nucleotides.

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Abstract
Pt binding to nucleoside phosphorothioates was examined to determine their suitability as heavy metal labeling sites for the potential EM sequencing of nucleic acids. The complex of platinum terpyridine nitrate forms a 1:1 adduct with adenosine or uridine monophosphorothioate. Spectroscopic evidence strongly indicates the presence of Pt-S bonds. Both platinum terpyridine nitrate and chloroterpyidineplatinum(II) bind to .**GRAPHIC**. a polymer prepared from adenosine 5''-O-(1-thiotriphosphate) and UTP. Binding to the sulfur atoms of the phosphorothioate groups is quantitative, as shown by double label experiments using .**GRAPHIC**. and [3H]chloroterpyridine-platinum(II). Similar experiments with [14C]poly(A-U) indicated no Pt binding. No evidence of nicking or S loss from .**GRAPHIC**. could be detected after Pt binding. The phosphorothioate group is a strong, highly selective binding site for Pt in polynucleotides. Previous studies have demonstrated quantitative enzymatic incorporation of phosphorothioate groups into a polynucleotide adjacent to a specific base. The use of heavy metal-lacked phosphorothioate groups for the sequencing of nucleic acids by EM therefore appears feasible.