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A 16S ribosomal RNA gene is a DNA sequence transcribed to produce16S ribosomal RNA, a structural and functional RNA of the small ribosomal subunit in bacteria and archaea. It is an RNA-coding gene, rather than a gene whose final product is a protein. The Escherichia coli rrsA annotation supplies a concrete example: NCBI records its gene type as rRNA and its product as16S ribosomal RNA.^[ref]

Sequence comparison makes the 16S ribosomal RNA gene useful as a marker of microbial relationships. The historical motivation includes a1977 analysis by Carl R. Woese and George E. Fox using ribosomal RNA sequence characterization to distinguish deep lineages. That experiment analysed RNA sequence information; it should not be described as a modern polymerase chain reaction survey.^[ref]

Gene copy number complicates abundance interpretation. A2004 genome comparison examined both variable copy numbers and sequence differences between16S copies in one genome.^[ref] In an invented ideal assay,100 cells with one target copy each contribute100 templates, while100 cells with four copies contribute400. Equal cell counts then give20percent and80percent of templates. Extraction, primer matching and amplification can introduce further differences. A sequence-read fraction is consequently not automatically a cell fraction.

Carl R. Woese (1928–2012) was a microbiologist who used ribosomal RNA comparisons to investigate deep evolutionary relationships. His work with George E. Fox helped establish that Archaea form a major lineage distinct from bacteria, despite both groups lacking a cell nucleus.^[ref]

Their 1977 paper inferred three primary lines of descent from ribosomal RNA sequence characterization.^[ref] The related experimental work compared catalogs of short fragments obtained from 16S ribosomal RNA by ribonuclease T1 digestion.^[ref] These were measurements of particular molecular sequences, rather than a classification based solely on visible cell shape.

A toy phylogenetics problem illustrates the motivation. Three rod-shaped microbes look alike, but a set of comparable molecular characters groups the first two together and separates the third. Cell shape alone would conceal that distinction. A molecular comparison can supply different evidence about shared ancestry, although interpreting it still requires an appropriate evolutionary model and enough informative characters.

The biological surprise was that methane-producing microbes could resemble ordinary bacteria at the level of broad cell organization while differing profoundly in the molecular evidence. Woese's approach made ribosomal RNA a way to test classifications of organisms whose appearance supplied few distinctive clues.^[ref]

The historical proposal is a foundation for the three-domain system. It does not settle every later question about the root and branching of the phylogenetic tree; the two-domain system addresses a different arrangement of the deepest relationships. Woese's enduring methodological contribution was to make such questions answerable through explicit molecular comparisons.

George E. Fox is a researcher in ribosome function, ribosomal RNA structure and molecular evolution. The University of Houston identifies him as a co-discoverer of Archaea and describes his laboratory's use of comparative analysis to investigate ribosomal RNA organization and the evolution of the ribosome.^[ref]

In 1977 Fox was first author of a study characterizing 16S ribosomal RNA from ten methane-producing microbial species. Digestion with ribonuclease T1 generated short fragments for ribosomal RNA oligonucleotide cataloguing. Comparing the catalogs revealed a distinct evolutionary group, distant from typical bacteria.^[ref] Fox and Carl R. Woese then jointly published the broader proposal distinguishing three primary lines of descent.^[ref]

The catalog comparison used an association coefficientwhere $N_A$ and $N_B$ count nucleotide residues represented by catalogued fragments of at least six nucleotides, and $N_{AB}$ counts the residues in their shared sequences. The weighting is by represented residues, rather than merely by the number of different fragment strings.^[ref]

For a constructed pair with $N_A=100$, $N_B=140$ and $N_{AB}=60$, the coefficient is $120/240=0.5$. Identical catalogs would give one, and no overlap would give zero. This arithmetic explains the comparison's normalization; the value is not directly a fraction of whole-genome identity or a calibrated divergence time.

Fox's work helped make evolutionary relationships among superficially similar microorganisms experimentally accessible through a shared molecular system, the ribosome.

Ribosomal RNA oligonucleotide cataloguing compares organisms through collections of short sequences obtained from ribosomal RNA. Instead of determining one complete modern sequence, investigators generated fragments, identified their sequences and compared which longer fragments were shared. The resulting catalog supplied molecular evidence about relationships beyond external appearance.

The 1977 Carl R. Woese and George E. Fox paper describes digestion of small-subunit ribosomal RNA with ribonuclease T1, followed by separation of the products using two-dimensional electrophoresis. Sequencing the resulting oligonucleotides produced a catalog characteristic of the examined material. Comparing catalogs helped establish the distinct position of the methanogenic organisms discussed in the paper. This was an RNA-fragment experiment, not a modern amplified-marker survey.^[ref]

The RNA oligonucleotide association coefficient used nucleotide counts in cataloged sequences of six or more residues:Here $N_A$ and $N_B$ count nucleotides represented by the eligible fragments in the respective catalogs, while $N_{AB}$ counts those represented in their shared portion. It is therefore not simply the fraction of fragment labels found in common, nor the percentage identity of a complete aligned RNA.^[ref]

For an original accounting example, let $N_A=40$, $N_B=60$ and $N_{AB}=25$. Then $S_{AB}=50/100=0.50$. A catalog with fewer recorded nucleotide residues can share the same amount of material while giving a different normalized comparison. Agreement in short fragments alone also need not imply identical full sequences.

The scientific advance was a reproducible molecular comparison usable across morphologically different microbes. Its interpretation still depended on the marker, sampling and method. A catalog similarity supplies evidence for analysis in phylogenetics, not a complete reconstruction of every gene, a direct date of divergence or a guarantee that two organisms have the same physiology.

The RNA oligonucleotide association coefficient measures the overlap of two catalogs produced through ribosomal RNA oligonucleotide cataloguing, weighting the retained shared fragments by their nucleotide content. In the 1977 comparison, catalogued sequences had at least six residues, andwhere $N_A,N_B$ count nucleotide residues represented in the two eligible catalogs and $N_{AB}$ counts those represented in their shared portion. It is a catalog-overlap statistic, not the percentage identity of a complete sequence alignment.^[ref]

An original example uses distinct fragment labels with specified lengths. CatalogA contains $a_6,b_8,c_{10}$; catalogB contains $a_6,b_8,d_6$. The subscripts denote nucleotide counts, and differently named fragments are different sequences. ThusCounting only matched labels would instead give two shared labels out of three per catalog, producing $2/3$. The difference exposes the effect of weighting: a shared eight-residue fragment contributes more than a shared six-residue fragment.

For a nonzero denominator, $0\le N_{AB}\le \min (N_A,N_B)$ gives $0\le S_{AB}\le 1$. A useful equivalent form isThe numerator in the subtracted fraction represents the catalog material outside the overlap under this counting model. If both catalogs are empty, the original formula has denominator zero; assigning a value would require an extra convention.

The coefficient is also sensitive to catalog coverage. Add the same new six-residue fragment to both example catalogs: the totals become30 and26, with20 shared, so the coefficient rises to $40/56=5/7$. Add it only toA instead, and the coefficient is $28/50=0.56$. These constructed changes alter the statistic without specifying any new evolutionary history.

The original ribosomal RNA oligonucleotide cataloguing experiments used comparisons of this kind as evidence for phylogenetics. Turning a similarity into a particular phylogenetic tree or a divergence date needs additional assumptions and evidence. The statistic itself records the specified molecular overlap.^[ref]