In this letter Khorana reported results for elucidating the genetic code he obtained by using intricate biochemical methods
to make long strands of RNA with predetermined, simple, repeating sequences such as UCUCUCU . . . , which he found coded for
a polypeptide consisting of serine-leucine-serine-leucine, etc.
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1965-04-15 (April 15, 1965)
Khorana, H. Gobind
Original Repository: Wellcome Library for the History and Understanding of Medicine. Francis Harry Compton Crick Papers
Thank you for your letter of the 6th of April. First, about the Gordon Conference, we should be able to pay $400.00 plus
the living expenses for the week. I certainly look forward to seeing you there.
I was on the point of writing to you about recent results. There is really a great deal that I want to tell you and I may
have to follow up with another letter. What Jim wrote to you is correct, but really when he was here the experiments hadn't
been finished and even now, the analysis of the peptides is not complete. In the meantime, the experiments on the incorporation
stimulated by poly UC, poly UG, poly AC and poly AG are on enclosed photographs l-4. They do show interdependence for two
amino acids in each set. Poly AG does incorporate arginine and glutamate. Arg is AGA, consistent with the results from homopeptide
synthesis (stimulated by poly AAG, photograph 5) and also GAG is glu. Both GAG and GAA triplets stimulate binding of glutamyl-sRNA.
AGA trinucleotide does not give any binding of arginyl-sRNA as I wrote before, but as you will see from the enclosed list
of codon assignments, other triplets stimulate binding of arginyl-sRNA.
So far as identification of copeptides is concerned, in addition to seryl-leucine, we have worked with presumed (glu-arg).
Tryptic digestion gives two to those peptides which we assume to be glu-arg and dimer of that. We should soon complete identification
by Edman degradation.
In case of (cys-val)n, we have done kinetics of H+ hydrolysis after oxidation to cysteic acid and we find as a major dipeptide
valyl-cysteic acid. We spent a long long time on this one -- but have now decided to be contented with kinetic hydrolysis
results. Havn't been able to make other techniques or even prototype work
We have also done a lot of binding studies and a list of results is given on the enclosed sheet. In particular, note the
enclosed photographs 6 and 7. Asp-sRNA binding is stimulated by poly AAG and this binding does not appear to be due to wrong
sequences in poly AAG. As seen on photograph 7, even the trinucleotides, especially GAA, stimulated binding although at higher
We are still trying to write at least part of the total work and would like to submit them to J. Mol. Biol. as soon as we
can and I would write to you again about this and, of course, send you copies right away.
H. Gobind Khorana
(1) Like to hear further your views on trying to get at total nonsense. We are now actively working on making repeating trio
and tetra sequences and with DNA polymerase making DNA, like polymers from them. Don't know yet how it.
(2) Re. your question, in copeptide synthesis, do other amino acids in in small amounts! As far as we know, they don't.
But it would be a big project to do this quantitatively and rigorously and we would have to start on this systematically.
You will have our present data on this soon, I hope.
(3) I don't know about Fresco;s results. I saw him recently and he merely said that his results agreed with ours but
that since deamination of (AU)n was not complete, he got other amino acids in too. We had tried deaminations of poly AU too
but hadn't results. At the moment, I am reluctant to accept the data unless I see the actual characterization of (IU)n.
(8) I am also enclosing oligo glutamate synthesis using Polu AAG. The radioactive bond between (Glu)2 and Glu turns up in
control and Doty's lab. tells me that they have see it too. Probably glutamyl-acids.
Codon assignments from binding and/or polypeptide synthesis
Amino Acid; Codons
Arg; AGA*, CGC, CGA
Glu; GAA,o GAGo
His; CAC,o CAU
Ileu; AUU, AUC, AUA
Lys; AAA,o AAG,o UAA
Thr; ACA,o ACU, ACC
Tyr; UAU, UAC
Valine; GUG,o GUU
This is a list put together last week April 8th and soon we should add to it a good deal.)
N.B. In the above assignments, those triplets without any notation, assigned by us from binding studies; those with o on
them, on the basis of, both, binding and polypeptide synthesis; those with *, on basis of polypeptide (homo and/or copeptide)
and fail to give binding.