Scott Classen: August 2009 Archives

Wen Zhang and Jack Dunkle in the Cate lab have a nice report out in the August 21, 2009 issue of Science describing their latest crystal structures of the E. coli ribosome with and without tRNA mimcs. These new structure shed light on the rachet-like action of the intact ribosome as it interacts with tRNA in the A, P, and E sites.

Zhang_Dunkle_Cate_Science.jpegAbstract:
Protein biosynthesis on the ribosome requires repeated cycles of ratcheting, which couples rotation of the two ribosomal subunits with respect to each other, and swiveling of the head domain of the small subunit. However, the molecular basis for how the two ribosomal subunits rearrange contacts with each other during ratcheting while remaining stably associated is not known. Here, we describe x-ray crystal structures of the intact Escherichia coli ribosome, either in the apo-form (3.5 angstrom resolution) or with one (4.0 angstrom resolution) or two (4.0 angstrom resolution) anticodon stem-loop tRNA mimics bound, that reveal intermediate states of intersubunit rotation. In the structures, the interface between the small and large ribosomal subunits rearranges in discrete steps along the ratcheting pathway. Positioning of the head domain of the small subunit is controlled by interactions with the large subunit and with the tRNA bound in the peptidyl-tRNA site. The intermediates observed here provide insight into how tRNAs move into the hybrid state of binding that precedes the final steps of mRNA and tRNA translocation.

Zhang W, Dunkle JA, Cate JHD. "Structures of the Ribosome in Intermediate States of Ratcheting" Science 21 August 2009: Vol. 325. no. 5943, pp. 1014 - 1017.
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Rambo_RNA.gifCover Illustration from the September 2009 issue of RNA: Crystal structure of the lysine riboswitch bound to lysine (Protein Data Bank code: 3d0u) Image details show RNA: ribbon; lysine: space-filling representation, red; iridium hexamine ions: space-filling representation, blue.

Abstract:
Riboswitches are metabolite-sensitive elements found in mRNAs that control gene expression through a regulatory secondary structural switch. Along with regulation of lysine biosynthetic genes, mutations within the lysine-responsive riboswitch (L-box) play a role in the acquisition of resistance to antimicrobial lysine analogs. To understand the structural basis for lysine binding, we have determined the 2.8 angstroms resolution crystal structure of lysine bound to the Thermotoga maritima asd lysine riboswitch ligand-binding domain. The structure reveals a complex architecture scaffolding a binding pocket completely enveloping lysine. Mutations conferring antimicrobial resistance cluster around this site as well as highly conserved long range interactions, indicating that they disrupt lysine binding or proper folding of the RNA. Comparison of the free and bound forms by x-ray crystallography, small angle x-ray scattering, and chemical probing reveals almost identical structures, indicating that lysine induces only limited and local conformational changes upon binding.

Garst AD, Héroux A, Rambo RP, Batey RT. "Crystal structure of the lysine riboswitch regulatory mRNA element." J Biol Chem. 2008 Aug 15;283(33):22347-51. Epub 2008 Jul 1.
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