RESUMO
The cellular organelles translating the genetic code into proteins, the ribosomes, are large, asymmetric, flexible, and unstable ribonucleoprotein assemblies, hence they are difficult to crystallize. Despite two decades of intensive effort and thorough searches for suitable sources, so far only three crystal types have yielded high-resolution structures: two large subunits (from an archaean and from a mesophilic eubacterium) and one thermophilic small subunit. These structures have added to our understanding of decoding, have revealed dynamic aspects of the biosynthetic process, and have indicated the strategies adopted by ribosomes for interacting between themselves as well as with inhibitors, factors and substrates.
Assuntos
Cristalografia por Raios X/métodos , Ribossomos/química , Ribossomos/ultraestrutura , Archaea/química , Proteínas de Bactérias/química , Microscopia Eletrônica , Modelos Moleculares , Ligação Proteica , Conformação Proteica , Estrutura Terciária de Proteína , RNA/química , Difração de Raios XRESUMO
Within the framework of ribosomal crystallography, the small subunits are being analyzed, using crystals diffracting to 3 A resolution. The medium resolution electron density map of this subunit, obtained by multiple isomorphous replacement, show recognizable morphologies, strikingly similar to the functional active conformer of the small ribosomal subunit. It contains elongated dense features, traceable as RNA chains as well as globular regions into which the structures determined for isolated ribosomal proteins, or other known structural motifs were fitted. To facilitate unbiased map interpretation, metal clusters are being covalently attached either to the surface of the subunits or to DNA oligomers complementary to exposed ribosomal RNA. Two surface cysteines and the 3' end of the 16S ribosomal RNA have been localized. Targeting several additional RNA regions shed light on their relative exposure and confirmed previous studies concerning their functional relevance.
Assuntos
RNA Ribossômico/química , Ribossomos/química , Cristalografia por Raios X , Cisteína/química , DNA Complementar/química , Substâncias Macromoleculares , Modelos Moleculares , Conformação de Ácido Nucleico , Conformação Proteica , RNA Bacteriano/química , RNA Ribossômico 16S/química , Proteínas Ribossômicas/química , Eletricidade Estática , Thermus thermophilus/químicaRESUMO
The small ribosomal subunit performs the decoding of genetic information during translation. The structure of that from Thermus thermophilus shows that the decoding center, which positions mRNA and three tRNAs, is constructed entirely of RNA. The entrance to the mRNA channel will encircle the message when a latch-like contact closes and contributes to processivity and fidelity. Extended RNA helical elements that run longitudinally through the body transmit structural changes, correlating events at the particle's far end with the cycle of mRNA translocation at the decoding region. 96% of the nucleotides were traced and the main fold of all proteins was determined. The latter are either peripheral or appear to serve as linkers. Some may assist the directionality of translocation.
