= Small ribosomal subunit
The <small ribosomal subunit> is the part of a <ribosome> that organizes <messenger RNA> and <transfer RNA> during decoding in <translation (biology)>. 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.<Molecular Biology of the Cell, 4th edition by Bruce Alberts and others>[From RNA to Protein, ribosomes and the roles of their two subunits]{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.<Molecular Biology of the Cell, 4th edition by Bruce Alberts and others>[From RNA to Protein, ribosome size and subunit composition]{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 $3\,\text{Å}$ resolution, gave a molecular framework for interpreting biochemical results about decoding and <ribosome> assembly.<Structure of the 30S ribosomal subunit by B. T. Wimberly and others>[Nature 407, 327–339; Abstract, structure determination and functional interpretation]{ref} A static structure establishes spatial organization; understanding the steps of <translation (biology)> additionally requires evidence about the moving, interacting complexes.
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