Influence of nucleotide sequence adjacent to duplex DNA termini on 3' terminal labeling by terminal transferase

We have analyzed the effect of base-pairing at the exposed ends of Hinc II fragments of SV40 DNA on the efficiency of ribonucleotide incorporation catalyzed by terminal transferase. Wide variations in the labeling efficiency of individual DNA fragments have been observed. To elucidate the nature of this variation at the molecular level, we have correlated this effect with nucleotide sequence adjacent to the cleavage site. We found that a G:C base pair right at the exposed end drastically reduces the incorporation of ribonucleotides. Furthermore, the higher the number of G:C base pairs adjacent to the exposed end, the greater the reduction in labeling efficiency. From these results, we conclude that the labeling efficiency is determined by the degree of 'terminal breathing' of the DNA molecule at the exposed end of the duplex.