Long pyrimidine tracts of L-cell DNA: localization to repeated DNA.

Like HeLa DNA, L-cell DNA contains a significant number of unexpectedly long pyrimidine tracts. 1.3% of the thymidine residues of L-cell DNA are found in these long polypyrimidine tracts. Analysis of L-cell DNA fractions with different rates of reassociation indicates that polypyrimidine tracts are associated with "repeated" DNA. Longer pyrimidine tracts appear to have a higher repetition frequency than shorter tracts, and some may be repeated as many as 5 x 10(4) times in L-cell DNA.