FINGERPRINT: X-ray film “fingerprints” of digested small subunit ribosomal RNAs, such as this one annotated by Carl Woese, led Woese to the discovery of the three domains of life.COURTESY OF NORMAN R. PACE, JAN SAPP, AND NIGEL GOLDENFELD. PNAS, 109:1011–18, 2012.In a letter to Francis Crick dated June 24, 1969, Carl Woese, a microbiologist at the University of Illinois, wrote that he wanted to use “the cell’s ‘internal fossil record’”— specifically, the RNA of the cell’s translation machinery—“to extend our knowledge of evolution backward in time by a billion years or so.” Soon enough, Woese’s team had RNA sequencing up and running in the lab.
The protocol consisted of digestion and two-dimensional electrophoresis of radioactive small-subunit (16S or 18S) ribosomal RNA. When exposed to X-ray film, the separated fragments generated a unique fingerprint, which Woese interpreted based on the position of the spots.
“Literally every single day he sat in front of those fingerprints and analyzed them,” says George Fox, a postdoctoral fellow in the Woese lab from 1973 to 1977 and now a professor of biology and biochemistry at the University of Houston. After secondary and tertiary digestion of the spots, Woese and Fox determined the sequences of the oligonucleotide fragments—each about 6 to 14 nucleotides long—and recorded them on 80-column IBM punch cards. The team then compared the catalog of sequences from each organism using a computer program ...
