Another piece of the ribosome: solution structure of S16 and its location in the 30S subunit

X-ray crystallography has recently yielded much-improved electron-density maps of the bacterial ribosome and its two subunits and many structural details of bacterial ribosome subunits are now being resolved. One approach to complement the structures and elucidate the details of rRNA and protein packing is to determine structures of individual protein components and model these into existing intermediate resolution electron density.

We have determined the solution structure of the ribosomal protein S16 from Thermus thermophilus. S16 is a mixed alpha/beta protein with a novel folding scaffold based on a five-stranded antiparallel/parallel beta sheet. Three large loops, which are partially disordered, extend from the sheet and two alpha helices are packed against its concave surface. Calculations of surface electrostatic potentials show a large continuous area of positive electrostatic potential and smaller areas of negative potential. S16 was modeled into a 5.5 A electron-density map of the T. thermophilus 30S ribosomal subunit.

The location and orientation of S16 in a narrow crevice formed by helix 21 and several other unassigned rRNA helices is consistent with electron density corresponding to the shape of S16, hydroxyl radical protection data, and the electrostatic surface potential of S16. Two protein neighbors to S16 are S4 and S20, which facilitate binding of S16 to the 30S subunit. Overall, this work exemplifies the benefits of combining high-resolution nuclear magnetic resonance (NMR) structures of individual components with low-resolution X-ray maps to elucidate structures of large complexes.