Crystals of the small ribosomal subunit from Thermus thermophilus appear as long, but thin needles of average dimensions of 80x80x300 μm3.
Prerequisite for structure determination is in many cases the incorporation of heavy atoms. In our case, giant tungsten compounds (so called W18-cluster) served as (heavy-)heavy atoms.
Position and occupation of several cluster binding sites varied substantially among different crystal types. The differences allowed to determine sufficient phase information to permit the interpretation of the electron density maps, i.e. to solve the structure ... (see xray-structure-analysis (so far only german, sorry)).
Most crucial und rather unexpected was however a side effect of these clusters: the stabilization of the crystal structure accompanied by a tremendous improvement of the diffraction to a resolution of 3Å. Only these high resolution crystals allowed us to interpret the electron density and hence to solve the structure (see PubMed).
Views of the structure
Facing the 50S subunit
+90o, Platform in front
The 30S ribosomal subunit from Thermus thermophilus by JMH
View from the back, facing the solvent
+90o, 'Shoulder' in Front
The structure of tje 30S ribosomal subunit from Thermus thermophilus shown from different view points (PubMed). Like in the case of the 50S subunit, the interface towards the other subunit is essentially free of ribosomal proteins. The most important part of the 30S subunit - the decoding site - is also exclusively composed of 16S rRNA nucleotides. The ribosomal proteins have nevertheless substantial influence on the accuracy and speed of the decoding process, which translates the genetic information contained on the mRNA.
The distribution of 16S rRNA domains within the 30S
Facing the 50S subunit
View from the back, facing the solvent
+90o, Platform in front
Spacial distribution of the 16S rRNA domains within the 30S subunit from T. th. by JMH
+90o, 'Shoulder' in front
The 2D structure of 16S rRNA allows a rather natural division into different domains. Contrary to 23S rRNA, the division of the secondary structure correlates well with the spatial distribution of the domains within the 30S subunit. The individual domains are fairly well separated, and most of the interactions between them are mediated by ribosomal proteins, which in a sense serve as a glue stabilizing the fold of 16S rRNA (and ultimately the 30S). .
The major advantage of the modular architecture - from a technical point - was a comparably easy interpretation of the electron density maps ;-) ... it wasn't that easy to get lost .... (JMH: Where am I? Model building inside virtual space would have been much easier)
Remarkable is the separation between the Head and the Body of the 30S subunit. Both structural elements are connected through a single rRNA-helix (H28), which permits small independent movements. This mobility is required for proper translocation of the mRNA by precisely one codon (or 3 nucleotides). Actually, some people prefer to visualize translocation as a gliding of the ribosome on the mRNA, rather than pulling the mRNA through the ribosome ... The small gap between the Head and the Shoulder serves as a kind of clamp, which allows to fix the position of the mRNA during the decoding process. After decoding (or peptide bond formation), the head rotates presumably together with the mRNA, which should induce a precise one-codon-translocation.
Protein distribution
Facing the 50S subunit
+90o, Platform in front
spatial distribution of ribosomal proteins within the 30S subunit from T. th. by JMH
, 
/