A small release of Pi from a diphenylamine-formic acid digest of DNA was detected after elimination of interpurine phosphodiester bonds was complete. Minor components in the DNA digest were identified as pyrimidine oligonucleotides which had lost one terminal phosphate. Isolated pyrimidine tracts released Pi on redigestion with the formic acid-diphenylamine reagent in amounts that increased with the number of nucleotides in the oligonucleotide taken. The oligonucleotides were also partially degraded by the formic acid-diphenylamine reagent and the degradation (2-3% of phosphodiester bonds between consecutive nucleotides) was almost independent of chain length. The cleavage was random with no preference for a phosphodiester bond flanked by particular nucleosides. This minor lack of specificity in the formic acid-diphenylamine-catalysed degradation of DNA can, however, account for the low recoveries of long pyrimidine tracts previously reported. Any analysis of pyrimidine tracts in a DNA molecule should make some correction for this small degree of degradation if exact assignments of the numbers of pyrimidine tracts are to be made.
