Small ribosomal subunit

The small ribosomal subunit is the part of a ribosome that organizes messenger RNA and transfer RNA during decoding in translation. A codon in the messenger RNA pairs with an appropriate anticodon in a transfer RNA. The large ribosomal subunit supplies the site for joining the delivered amino acids into the growing protein. Both ribosomal subunits are required for the complete translating machine.^[ref]

In bacteria, the small ribosomal subunit is conventionally called 30S. It contains ribosomal RNA and ribosomal proteins. The S labels refer to sedimentation coefficients, not additive masses: a bacterial 30S small ribosomal subunit and 50S large ribosomal subunit form a 70S ribosome. Their combined shape and resistance to motion affect sedimentation, so ordinary addition of the labels is inappropriate.^[ref]

An invented messenger RNA example illustrates why positioning matters. Grouping the sequence AUGGCUAAU in one reading frame gives AUG|GCU|AAU. Starting one nucleotide later instead gives UGG|CUA|.... The sequence has not changed, but the codons presented for decoding have. The small ribosomal subunit participates in organizing the messenger RNA and transfer RNA interactions needed to maintain the appropriate reading frame.

A major structural advance was the 2000 determination of the bacterial 30S ribosomal subunit structure by B. T. Wimberly and collaborators using X-ray crystallography. Their model, refined to $3\text{A˚}$ resolution, gave a molecular framework for interpreting biochemical results about decoding and ribosome assembly.^[ref] A static structure establishes spatial organization; understanding the steps of translation additionally requires evidence about the moving, interacting complexes.