16S rRNA, 18S rRNA, 70S and 80S Ribosomes: What Does the “S” Stand For?
Aharon Oren (Microbe, 2015)
Sequence data of small subunit rRNA (16S rRNA for prokaryotes, 18S rRNA for eukaryotes) are an integral part of every paper describing new species of microorganisms. And because of the availability of extensive ribosomal RNA sequence databases, most “cultivation-independent” studies of microbial communities in natural environments target 16S or 18S rRNA genes. I estimate that 16S or 18S rRNA genes feature in at least 70–80% of all papers in journals such as FEMS Microbiology Ecology, ISME Journal, and Microbial Ecology.
Although the great majority of microbiologists use the terms 16S and 18S rRNA daily, hardly anybody appears to know what the “S” stands for. To prove this fact I devised a little quiz. The question asked was: “What is the dimension of the “S” unit (as used in 16S rRNA, 70S ribosomes, etc.) in SI Units (kg = kilogram; m = meter, s = second)”. I presented the following options: (1), Dimensionless; (2), kg; (3), m; (4), s; (5), kg/m; (6), kg/m2, (7) kg/m3; (8), m/s; (9), m/s-2; (10), kg.m/s; (11), kg.m/s2; (12), kg/m3.s; (14), kg/s; (15), kg.s/m3; (16), No idea. I presented this quiz at four different occasions: (i) 16 April 2015, The Winogradsky Institute of Microbiology, Moscow, Russia, for an audience of 20 senior scientists and 40 graduate and senior undergraduate students; (ii) 15 May 2015, The Interuniversity Institute for Marine Sciences in Eilat, Israel, during a course marine microbiology for 20 M.Sc. and Ph.D. students and their teachers; (iii) 8 June 2015, at a session on “The Species Concept in the Genomic Era” during the 6th FEMS congress in Maastricht, The Netherlands, attended by 90–100 participants; (iv) 28 August 2015 at the 7th National Conference of Microbial Resources & the International Symposium on Microbial Systematics and Taxonomy, Hangzhou, China, where 310 scientists and students were present.
The result was interesting: out of the total of almost 500 participants in the quiz, nearly all of which were scientists and students who are almost daily confronted with the terms 16S rRNA etc., only one (in Maastricht) give the correct answer which is option 4: the S (= Svedberg) is a unit of time and is therefore measured in seconds. Except for option 16 (“No idea”), popular answers were “Dimensionless” and “m/s2”. Once or twice someone hesitantly commented that it may have something to do with sedimentation. Indeed the Svedberg unit is the ratio between the speed of a particle in the ultracentrifuge (measured in m/s) and the centrifugal force (m/s2), 1 S being equivalent to 10(-13) s.
While biochemistry textbooks give in-depth information on the use of the analytical ultracentrifuge, most microbiology textbooks provide little explanation. Some textbooks do not explain the nature of the S unit (e.g., B. D. Davis et al., Microbiology, 4th ed., 1990; A. Salyers and D. Whitt, Microbiology. Diversity, Disease, and the Environment, 2001). The connection between the S unit and sedimentation is given by M. T. Madigan et al., Brock Biology of Microorganisms, 13th ed., 2012 (“The S-values are Svedberg units, which refer to the sedimentation coefficients of ribosomal subunits (30S and 50S) or intact ribosomes (70S) when subjected to centrifugal force in an ultracentrifuge”). The most accurate description I found in a modern microbiology textbook is “The rate of sedimentation per unit of centrifugal force is called the sedimentation coefficient and is generally expressed in Svedberg (S) units” (R. Atlas, Principles of Microbiology, 2nd ed., 1997). I did not read the definition of 1 S = 10(-13) s in any of the microbiology textbooks I checked. Thus it may not be too surprising that so few microbiologists know the meaning of a basic unit they use every day.
