title: Conformation of viroids.
abstract: Viroids are uncoated infectious RNA molecules (MW 107 000-127 000) known as pathogens of certain higher plants. Thermodynamic and kinetic studies were carried out on highly purified viroid preparations by applying UV-absorption melting analysis and temperature jump methods. The thermal denaturation of viroids is characterized by high thermal stability, high cooperativity and a high degree of base pairing. Two relaxation processes could be resolved; a process in the sec range could be evaluated as an independent all-or-none-transition with the following properties: reaction enthalpy= 550 kcal/mol, activation enthalpy of the dissociation = 470 kcal/mol; G : C content = 72 %. These data indicate the existence of an uninterrupted double helix of 52 base pairs. A process in the msec range involves 15 - 25 base pairs which are most probably distributed over several short double helical stretches. A tentative model for the secondary structure of viroids isproposed and the possible functional implications of their physicochemical properties are discussed.
mesh_terms: D055598:Chemical Phenomena; D002621:Chemistry; D003597:Cytosine Nucleotides / Q000032:analysis; D006150:Guanine Nucleotides / Q000032:analysis; D007700:Kinetics; D009690:Nucleic Acid Conformation; D009691:Nucleic Acid Denaturation; D010935:Plant Diseases; D010944:Plants; D012313:RNA; D012265:Ribonucleotides / Q000032:analysis; D013696:Temperature; D013816:Thermodynamics; D014508:Urea
chemical_list: D003597:Cytosine Nucleotides; D006150:Guanine Nucleotides; D012265:Ribonucleotides; D012313:RNA; D014508:Urea