October 17, 1961.
Dear Dr. Berg,
I would greatly appreciate having one copy each of your four recent papers entitled "The Enzymic Synthesis of Amino Acyl Derivatives of Ribonucleic Acid," J. Biol. Chem. 236, 1726, 1735, 1741, 1748 (1961).
In the past year we have been measuring [alpha]-glutamyl RNA, glutaminyl RNA and glycyl RNA synthetase activities in rat liver fractions by following the rate of labeling of sRNA in heated rat liver "pH5 enzymes" on addition of very small amounts of partially purified fractions in the presence of excesses
of ATP and the appropriate C14-amino acid.  Under these conditions the rate of labelling of sRNA is proportional to enzyme concentration as you have reported in Paper I.  These activities have proven, up to now, somewhat refractory to
measurement by the usual hydroxamate and ATP-PP32 exchange tests, although in partially purified preparations we have obtained both glutamate- and glutamine-dependent ATP-PP32 exchanges.
One thing that has been essential for us in the measurement of the [alpha]-glutamyl RNA and glutaminyl RNA synthetase activities.  That has been elimination of C 14-glutamate C14- glutamine interconversion by the addition of a glutamine synthetase inhibitor, methionine sulfoximine. If this is not added then both [alpha]-glutamyl- and glutaminyl-RNA's form in the "cruder" preparations as identified by hydrolysis and ammonolysis of the labeled SRNA. This brings me to a question concerning your experiments that indicated that the polynucleotide chains specific for accepting L-methionine are heterogeneous.  Is it not possible that the E. coli methionyl RNA synthetase preparation contains enzymes which convert methionine to some other amino acid (e.g. cysteine) and
catalyze the formation of another amino acyl RNA?  Have you identified the product(s) formed by both the E. coli and yeast methionyl RNA synthetase preparations (separately and
Yours sincerely,
Murray J. Fraser
Murray J. Fraser, Ph.D.
Assistant Professor of Biochemistry
Enclosed are the reprints you requested.  I was happy to hear of your results with glutamyl-, glutaminyl-, and gycyl RNA [. . .].  Your suggestion to an alternative interpretation [. . .] data is a good one.  I don't think we can eliminate by any of our data [. . .] skeptical that [. . .] will convert methionine to cysterine without any additions ever if all the [. . .] necessary for the conversion were present.
Sin.
PB