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Abstract

A new procedure for determining DNA nucleotide sequences is reported. In the first step of the method, four DNAs, each separately substituted with a different deoxynucleoside phosphorothioate in place of the corresponding monophosphate, are prepared by template-directed polymerization catalyzed by DNA polymerase. In the second step, these DNAs are subjected to stringent exonuclease III treatment, which produces only fragments terminating with a phosphorothioate internucleotide linkage. These can then be separated by standard gel electrophoresis techniques and the sequence can be read directly as in presently used sequencing methods.