Repeated sequences in L-cell mRNA complementary to long deoxypolypyrimidines

Abstract

Long pyrimidine tracts form part of the repeated DNA sequences in eukaryotic genomes and are among the most highly conserved sequences in evolution. At least some of these sequences are transcribed in L-cells, since they are capable of hybridizing extensively with total cellular RNA. In saturation hybridization experiments involving fractionated cellular RNA, polypyrimidines react preferentially with the polysomal poly(A+) RNA and the nuclear poly (A+) RNA fractions; there is little or no reaction with poly (A-) RNA or with synthetic poly (A). In addition, hybridization between mRNAs and deoxypolypyrimidines is not affected by enzymatic removal of the poly (A+) tract from mRNA molecules. Saturation hybridization experiments indicate that about 1% of mRNA consists of regions complementary to deoxypolypyrimidines. The size of the polypurine regions in mRNA has been measured by gel electrophoresis of RNAase-resistant hybrids and was found to be heterogeneous with an average size of about 35 nucleotides. It is estimated that about half of poly (A-) mRNA molecules in L-cells contain sequences complementary to deoxypolypyrimidines. Although the function of polypyrimidine tracts is not yet known, the conservation and transcription of these sequences suggests an important role in the expression of eukaryotic genes.