A Quantitative Model of the Escherichia coli 16 S RNA in the 30 S Ribosomal Subunit
We use a computer-based protocol for automated structure refinement of large RNAs and ribonucleoproteins to propose a three-dimensional model for the Escherichia coli 16 S RNA in the 30 S ribosomal subunit along with the first quantitative estimates of the uncertainties in the model. Our models are based on the 16 S RNA secondary structure, the small angle neutron scatter map of 30 S proteins, tertiary RNA-RNA and RNA-protein contacts as suggested by cross-linking, chemical footprinting and other experimental studies, and electron microscopy data for the shape of the 30 S subunit and placement of 16 S RNA fragments, along with known motifs in RNA structure. In addition, some data on the interaction of the tRNAs/mRNA with the 16 S RNA were used to localize the active site. Since there are not enough structural data to derive a unique three-dimensional folding of the 16 S RNA, several different conformations can be generated to satisfy the experimental data. A set of seven models was refined to survey the range of acceptable conformations. These models were analyzed to deduce probable positions and orientations of the different helical segments that comprise the 16 S RNA in the Escherichia coli small subunit, and one consensus model from this set is presented here. An estimate of the reliability of our predicted structure is made using the variations between the models, and about 75% of 16 S RNA helical segments are localized to 15 A or less in their position in the small subunit. Our models show a distinct separation of the three major domains of the 16 S RNA. The 5' major domain and the central domain are clustered in the body of the 30 S subunit, whereas the 3' major domain is localized in the head of the subunit. Our modeling results are compared with models of the 16 S RNA proposed by other researchers, and are seen to be similar to the manually built models by Stern et al. and Brimacombe et al. with a few significant differences. The position of nucleotides implicated by footprinting and crosslinking data in tRNA and mRNA binding, and in subunit association are examined, and many of these sites are seen to lie along the 30 S subunit neck, cleft and the platform.