The 3′-Terminal Heptanucleotide Sequence of the l Strand of T7 Deoxyribonucleic Acid

In a reaction catalyzed by the T4 DNA polymerase, the 3'-terminal deoxythymidylate of the l strand of T7 DNA can be exchanged for a [32P]deoxythymidylate without introducing any net covalent changes into the molecule. Degradation of the terminally labeled [32P]DNA to pyrimidine oligonucleotides using diphenylamine and formic acid revealed that almost all of the radioactivity was in a unique pyrimidine pentanucleotide. This fact indicates that the l strand has a 3'-terminal sequence of five pyrimidines. By 5'-terminal labeling and exonuclease digestion of the purified pentanucleotide, its structure was shown to be d-pTpCpCpCpT. This structure, together with data obtained earlier from nearest neighbor analyses (Englund, P. T. (1972) J. Mol. Biol. 66, 209–224), indicates that the 3'-terminal nucleotide sequence of the l strand of T7 DNA is ... pTpGpTpCpCpCpT.