HeLa cell nuclear DNA contains an unexpectedly large amount of long pyrimidine polynucleotides.These sequences were detected in DNA labeled with ['HI thymidine by treatment with formic acid-diphenylamine and subsequent analysis by polyacrylamide gel electrophoresis. About 0.5% of the total thymine residues were found in polynucleotides that migrated more slowly than 4S RNA. No polynucleotides of comparable size were detected in Escherichia coli DNA. The pyrimidine polynucleotides contained no pure poly(dT) sequences (less than 0.0015% of the total residues), as judged by thermal chromatography of a complex formed with poly(A).When high molecular weight DNA is treated with acid under relatively mild conditions (1, 2), purine bases are selectively removed, the phosphodiester backbone is broken, and pyrimidine clusters having the general formula, p(Pyp)5, are released. The largest pyrimidine oligonucleotides previously isolated from animal cell DNA contain 15-20 nucleotides (3)(4)(5). Small amounts of longer polynucleotides have been detected but not defined. Because relatively long polyadenylate segments (6-11) and uridylate-rich regions (12) have been found in cellular RNA, we have examined pyrimidine polynucleotides in HeLa cell nuclear DNA to see if corresponding sequences could be detected. Pyrimidine polynucleotides were separated according to chain length by polyacrylamide gel electrophoresis and, unexpectedly, a large amount of long polynucleotides was found. Since their size and amount differ very greatly from that predicted assuming a random distribution of pyrimidines, they may represent sequences of special significance in the genome. When annealed with commercial poly(A), almost no pure poly(dT) sequences could be detected.
MATERIALS AND METHODSPreparation of ['HIDNA. HeLa cells used in these experiments were grown in spinner culture in Eagle's minimal essential medium with 5% calf serum without antibiotics. 400 ml of cell suspension (3 X 105/ml) were incubated with [methyl-'H ]thymidine at a concentration of 1 gCi/ml, 0.5 AM, for 24 hr. Nuclei were isolated as described by Penman (13); the nuclear pellet was suspended in 0.5 ml of hypotonic buffer (10 mM NaCl-10 mM Tris HCl-3 mM MgCl2) and rapidly mixed with 8 ml of lysing solution [1 M LiCl, 0.1 M EDTA, 0.1 M Tris HCl (pH 8.0), and 1% Na dodecyl sulfate]. The suspension was heated at 600 for 5 min, then cooled to room temperature and passed twice through a no. 26 needle to reduce its viscosity. The preparation was deproteinized by shaking at room temperature for 30 min with an equal 2189 volume of chloroform containing 0.1 ml n-octanol. The aqueous phase was recovered by centrifugation and the procedure was repeated three times more. DNA was precipitated with ethanol and dissolved in 2 ml of a solution containing 10 mM NaCl, 1 mM EDTA, 1 mM Tris -HCl (pH 7.4), and 0.2% Na dodecyl sulfate. NaOH was added to 0.15 N, and the DNA was heated at 600 for 10 min to hydrolyze RNA. The sample was neutralized with 3 N acetic acid, and the DNA was pre...