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Sequences complementary to cellular deoxypolypyrimidines are localized in the 3' end of L-cell mRNA.

Many eukaryotic genomes have been shown to contain long pyrimidine tracts. In mouse L-cells, at least some of these pyrimidine tracts are transcribed and form a significant portion of the poly(A)+ RNA sequences. In this study the results from three different experimental methods indicate that the sequences complementary to polypyrimidines are localized to the 3' end of mRNA molecules. First, polypyrimidines reacted preferentially with the 3'-end fragments of mRNA generated by limited alkaline cleavage. Second, digestion of [3H]mRNA-polypyrimidine hybrids with RNase H released 3'-end fragments of mRNA which averaged only 250 nucleotides (NT) in length. Third, polypyrimidine tracts were isolated from cDNA which averaged only 200-250 NT extending from the 3' end of the corresponding mRNAs. These data suggest that the sequences transcribed from pyrimidine tracts are quite close to the 3' end of the cellular messages in which they occur, probably within the terminal untranslated region.