= 16S ribosomal RNA gene
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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.<rrsA in NCBI Gene>[GeneID948332, Summary, gene type and RNA name]{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.<Phylogenetic structure of the prokaryotic domain: the primary kingdoms by Carl R. Woese and George E. Fox>[Abstract and original-paper title/byline]{ref}
<Gene copy number> complicates abundance interpretation. A2004 genome comparison examined both variable copy numbers and sequence differences between16S copies in one <genome>.<Divergence and redundancy of 16S rRNA sequences in genomes with multiple rrn operons by Silvia G. Acinas and others>[Abstract and Introduction]{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.
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