There are three kinds of rRNA sequences in bacteria, 23S, 16S and 5S. Among them, 16S rRNA is the most commonly used molecular clock in the systematic taxonomic study of bacteria due to its moderate nucleotide number, a large amount of information, high stability and easy extraction and analysis.16S rDNA is about 1540bp in length and exists in all bacterial chromosome genomes.

16S rRNA has a large content (about 80% of bacterial RNA content). The evolution of 16S has highly conservative in structure and function, known as bacterial fossil. In most prokaryotes, rDNA has multiple copies with the same copy number in 5S, 16S, and 23S rDNA. The 16S rRNA contains 9 variables and 10 constant regions. The conservative sequence region reflects the genetic relationship between biological species, while the highly variable sequence region can reflect the differences between species. Because of its moderate size, 16S rDNA can use sequencing technology to obtain its sequence easily, so it is accepted by bacteriologists and taxonomists.

The sequence characteristics of 16S rDNA molecules have changed the basis of molecular biology for the phylogenetic classification of related species of different taxonomic levels. At present, 16S rDNA sequence information has been widely distributed for bacterial identification and phylogenetic research.