title: Physical studies of the interaction of a calf thymus helix-destablizing protein with nucleic acids.
abstract: UP1, a calf thymus protein that destabilizes both DNA and RNA helices, dramatically accelerates the conversion of the inactive conformers of several small RNA molecules to their biologically active forms [Karpel, R. L., Swistel, D. G., Miller, N. S., Geroch, M. E., Lu, C., & Fresco, J. R. (1974) Brookhaven Symp. Biol. 26, 165-174]. Using circular dichroic and spectrophotometric methods, we have studied the interaction of this protein with a variety of synthetic polynucleotides and yeast tRNA3Leu. As judged by perturbations in polynucleotide ellipticity or ultraviolet absorbance, the secondary structures of the single-stranded helices poly(A) and poly(C), as well as the double-stranded helices poly[d(A-T)] and poly(U.U), are largely destroyed upon interaction with UP1 at low ionic strength. This effect can be reversed by an increase in [Na+]: half the UP1-induced perturbation of the poly(A) CD spectrum is removed at 0.05 M Na+. The variation of poly(A) ellipticity and ultraviolet absorbance with [UP1]/[poly(A)]p is used to determine the length of single-stranded polynucleotide chain covered by the protein: 7 +/- 1 residues. A model is presented in which the specificity of UP1 for single strands and their concomitant distortion are a consequence of maximal binding of nucleic acid phosphates to a unique matrix of basic residues on the protein. Analogous to the effect on polynucleotides, UP1-facilitated renaturation of yeast tRNA3Leu follows the partial destruction of the inactive tRNA's secondary structure. At the tRNA absorbance maximum, UP1 effects a hyperchromic change of 10%, representing one-third of the secondary structure of the inactive conformer. This change is also clearly observable as a perturbation of the tRNA's circular dichroism spectrum.
mesh_terms: D000818:Animals; D002417:Cattle; D002942:Circular Dichroism; D004265:DNA Helicases / Q000378:metabolism*; D000076245:Heterogeneous Nuclear Ribonucleoprotein A1; D034481:Heterogeneous-Nuclear Ribonucleoprotein Group A-B; D008274:Magnesium / Q000494:pharmacology; D009690:Nucleic Acid Conformation; D011061:Poly A / Q000378:metabolism*; D011066:Poly C / Q000378:metabolism*; D011072:Poly U / Q000378:metabolism*; D011067:Poly dA-dT / Q000378:metabolism*; D011089:Polydeoxyribonucleotides / Q000378:metabolism*; D011131:Polyribonucleotides / Q000378:metabolism*; D012343:RNA, Transfer / Q000096:biosynthesis; D012261:Ribonucleoproteins; D012964:Sodium / Q000494:pharmacology; D013056:Spectrophotometry, Ultraviolet; D013096:Spermine / Q000494:pharmacology; D013951:Thymus Hormones / Q000378:metabolism*
chemical_list: D000076245:Heterogeneous Nuclear Ribonucleoprotein A1; D034481:Heterogeneous-Nuclear Ribonucleoprotein Group A-B; D011089:Polydeoxyribonucleotides; D011131:Polyribonucleotides; D012261:Ribonucleoproteins; D013951:Thymus Hormones; D011061:Poly A; D011067:Poly dA-dT; D011072:Poly U; D013096:Spermine; D011066:Poly C; D012343:RNA, Transfer; D012964:Sodium; D004265:DNA Helicases; D008274:Magnesium