Discovery of Transfer RNA Recounted
In the Sept. 16 issue of Nature,1 Mahlon Hoagland recounts how he did the key experiment in 1957 that proved DNA used “soluble RNA” intermediates, later named transfer RNA (tRNA), on the way to protein synthesis in the ribosome, only to find that Francis Crick had predicted the existence of such intermediates.
By this time , scientists generally believed that RNA copies of single strands of DNA, acting as templates prescribing the sequences of amino acids in proteins, existed on ribosomes. Frances Crick turned his attention to how amino acids might be ordered on such presumed templates. As there is no chemical similarity or complementarity between amino acids and nucleotides, and thus no means by which they could directly interact, Crick suggested that amino acids might be first attached to short single strands of RNA nucleotides, thereby making the amino acids ‘recognizable’ to complementary sequences of nucleotides on the templates. In its simplest form, 20 specific enzymes would catalyse the attachment of 20 different kinds of amino acids to 20 different RNA ‘adaptor’ molecules. These would then be ordered by complementary nucleotide pairing on single-stranded RNA templates on ribosomes. Francis circulated this ‘adaptor hypothesis’ among 20 fellow molecular biologists of the RNA Tie Club in 1955, but it was not formally published until 1958. (Emphasis added in all quotes.)
Prior to this, biochemists had considered the soluble RNA just “junk” in the mix of ribosomal RNA molecules. As the picture of transfer RNA emerged (including the discovery of the 20 additional enzymes, named aminoacyl-tRNA synthetases, that arm the tRNAs with their cognate amino acids), it looked like a wondrous design. Hoagland describes his delight at the time, miffed somewhat at having been scooped by Crick:
An image arose before me: we explorers, slashing and sweating our way through a dense jungle, rewarded at last by a vision of a beautiful temple – looking up to see Francis, on gossamer wings of theory, gleefully pointing it out to us!
And so it was that tRNAs and their companion activating enzymes (which came to be known as aminoacyl-tRNA synthetases), framed by the adaptor hypothesis, brought the classical biochemists and the molecular biologists together, snug in the same discipline, all speaking the same language.
1Mahlon Hoagland, “Turning Points: Enter transfer RNA,” Nature 431, 249 (16 September 2004); doi:10.1038/431249a.
This story forms a good example of how the intelligent design approach is good for science. Notice, first of all, how the wrong approach was to consider the soluble RNA as “junk.” An ID scientist would think instead that these molecules are there for a purpose and have some role to play. It took a pursuit based on belief in design to find the truth. For Hoagland, the pursuit was empirical: observing what actually happened. For Crick, it was theoretical: investigating how things should happen inside the black box, given the DNA template in the nucleus and the protein chain in the ribosome. The adaptor hypothesis was a “brilliant imaginative leap,” Hoagland calls it, because it reasoned that an underlying design was required to produce an ordered result.
The personal beliefs of the scientists about evolution or creation did not enter into the picture as long as they used the logic of intelligent design in their approach: effects must have a cause, and design is empirically detectable apart from one’s religious beliefs. The key to Crick’s insight was the realization that nucleotides and amino acids, having no chemical affinities, must become recognizable to one another during the process of translation. Notice how that word recognizable implies design, in the same sense a programmer designs a printer driver to enable the computer to recognize it. Without the driver interface, the computer and the printer would have no natural affinity. Transfer RNA and its synthetases form a complex suite of adaptors or translators that, like interpreters, understand two languages, the language of nucleotides and the language of proteins (see 06/09/2003 and 04/29/2003 headlines). Although it is unknown Crick reasoned this way at the time, the logical inference based on uniform experience is that anything that can translate one code into another must have had an intelligent cause. Perhaps this is one of the reasons Crick later became a proponent of the panspermia hypothesis. Since life appears too complex to have formed by chance, it must have been put here by intelligent designers. One would only wish Crick, who died in July, had reasoned further that they, too, must have been designed, and continued his reasoning back to an uncaused First Cause.
The discovery of transfer RNA gives us two lessons in the value of intelligent design in science. First (despite his personal evolutionary philosophy), Crick’s “adaptor hypothesis” presupposed an inherent design in the process of DNA translation, and Hoagland’s experiments presupposed a function for what others were calling junk. Second, the tRNA and the aminoacyl-tRNA synthetase families provide prima facie evidence of intelligent design by their ability to translate one coded information storage system into another, resulting in information-rich functional machinery. A picture is worth a thousand words. Watch tRNA at work in the film Unlocking the Mystery of Life. The animation of DNA translation and protein synthesis provides a five-minute, permanent cure for hallucinations caused by tripping out on Charlie’s Angle Dust (see 09/12/2004 commentary).