title: Association of an RNA kissing complex analyzed using 2-aminopurine fluorescence.
abstract: The fluorescent probe, 2-aminopurine-2'-O-methyl riboside (2-AP) has been selectively incorporated at adenosine positions in stem-loops (so called R1inv and R2inv), derived from the ColE1 plasmid encoded RNA I and RNA II transcripts, that interact to form stable loop-loop kissing complexes and bind the RNA one modulator (Rom) protein, such that fluorescence-detected stopped-flow and equilibrium methods could be used to study the detailed mechanism of this RNA-RNA interaction. Formation of loop-loop kissing complexes between R1inv and R2inv hairpins, substituted with 2-AP at positions in the complementary loops, results in a 5-10-fold fluorescence emission decrease (F(max) = 370 nm), which provides a sensitive measure for the binding reaction. The 2-AP substituted complexes are found to have equilibrium binding properties (average K(D) = 2.6 +/- 1.7 nM) and affinity for Rom (average K(D) = 60 +/- 24 nM) that are similar to complexes formed with equivalent unlabeled hairpins. Using stopped-flow experiments, it was found that the 2-AP probes experienced at least three different microenvironments during association of the RNA complex, thus suggesting a kinetic intermediate in the kissing pathway. In contrast, dissociation of the complex was found to fit a single exponential decay (average k(off) = 8.9 x 10(-5) s(-1)). Consistent with these observations, a two-step mechanism for RNA loop-loop complex association is proposed in which the complementary loops of R1inv and R2inv first base pair to form the loop-loop helix (average k(1) = 0.13 microM(-1)s(-1)) in the initial encounter reaction, and subsequently isomerize to the final tertiary fold in a second slower step (average k(2) = 0.09 s(-1)), where the helical stacking around the junctions is optimized.
authors.affiliation: Center for Advanced Research in Biotechnology of the University of Maryland and the National Institute for Standards and Technology, 9600 Gudelsky Drive, Rockville, MD 20850, USA.
authors.forename: M
authors.initials: M
authors.lastname: Rist
authors.forename: J
authors.initials: J
authors.lastname: Marino
mesh_terms: D015075:2-Aminopurine / Q000737:chemistry*; D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism; D005453:Fluorescence; D007700:Kinetics; D009690:Nucleic Acid Conformation; D011485:Protein Binding; D012329:RNA, Bacterial / Q000737:chemistry* / Q000378:metabolism; D013050:Spectrometry, Fluorescence
chemical_list: D001426:Bacterial Proteins; D012329:RNA, Bacterial; C042833:ROM protein, Bacteria; D015075:2-Aminopurine