Physical characterization of a ribosomal nucleoprotein complex

Abstract

The complex between ribosomal protein L24 and its RNA binding site (that region of the 23S RNA which the protein protects from ribonuclease digestion) [of Escherichia coli] was studied by various physicochemical methods. The RNA is composed of 2 fragments of .apprx. 160 and 140 nucleotides which interact with each other to form the L24 binding site. Circular dichroism spectroscopy suggests that the 2 interacting fragments have a unique region of secondary structure which is not present in either of the 2 components alone; hence there are important structural interactions between regions of the RNA which are separated in e primary sequence. Addition of the L24 protein to the RNA site promotes a structural change associated with base unstacking, but with little or no change in the H-bonded base pairing. Heat activation is not required for complex formation. Thermal denaturation studies reveal a broad featureless transition and the amount of hypochromic change indicates that the RNA site contains less secondary structure than other RNAs such as tRNA and total rRNA. Temperature-jump relaxation measurements on the mechanism of unfolding of the RNA show a concerted melting of the entire secondary and teritary structure, which is altered upon addition of the protein. A structural basis for the RNA-protein complex is discussed.