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16S ribosomal RNA is the ribosomal RNA of the small ribosomal subunit in bacteria and archaea. Together with ribosomal proteins, this RNA forms the small subunit's architecture, which positions messenger RNA for translation. The name refers to a sedimentation coefficient of about16 svedbergs: it is neither a count of subunits nor a length in nucleotides. The RNA molecule and the DNA gene encoding it are different molecules, even when a DNA sequencing assay is informally described as “16S sequencing”.^[ref]

A structural example is the bacterial30S small ribosomal subunit: the2000 X-ray crystallography study of B. T. Wimberly and colleagues resolved its RNA–protein architecture at3Å resolution. At a different scale, 16S ribosomal RNA sequences can serve as inherited characters for phylogenetic analysis. The1977 study by George E. Fox and colleagues compared ribosomal RNA oligonucleotide cataloguing results from methane-producing organisms with those from other organisms, helping expose a deep evolutionary division.^[ref][ref]

A useful caution is that one genome need not contain just one16S gene, and its copies need not have identical nucleotide sequences. Silvia G. Acinas and colleagues explicitly measured this variation across sequenced genomes. As a hypothetical example, two distinct16S DNA sequences recovered from a sample could represent two organisms, or two copies in one organism. Counting distinct sequences therefore requires more interpretation than counting organisms.^[ref]

The 30S ribosomal subunit is the bacterial small ribosomal subunit, an assembly containing 16S ribosomal RNA and ribosomal proteins. It binds messenger RNA and helps position transfer RNA anticodons relative to the message during biological translation. The large ribosomal subunit supplies the peptidyl transferase center; a small subunit alone is not a complete elongating ribosome.^[ref]

The 30S label refers to a sedimentation coefficient, not a count of 30 molecules or a molecular mass of 30 units. A svedberg is a unit of that transport coefficient. Shape, mass and interaction with the surrounding liquid all affect sedimentation.^[ref]

A short reading-frame example explains the subunit's positioning role. Grouping AUGAAACCC from its first base gives AUG|AAA|CCC. Shifting the grouping by one base gives UGA|AAC|CC, with an incomplete final group. The nucleotide inventory is unchanged, but the triplets presented for decoding differ. Position and pairing therefore matter alongside simple RNA attachment.

30S initiation complex experiments examine an assembled state involving the subunit, message and initiator transfer RNA. Reconstituting the subunit with or without ribosomal protein S1 has separated message binding from productive placement in the decoding channel.^[ref]