June 18, 1969
Dear Dr. Chargaff:
I read your interesting paper in the March issue of the PNAS.  There is in fact another study of exactly the sort you describe "binding between ribopolymer and the complementary mononucleotide in the presence of an enzyme acting on the latter".  The study is by Leon Heppel (JBC 238: 357, 1963) and deals with polynucleotide phosphorylase, and all possible base pairs.
In recent experiments with Dr. Robb Moses we confirmed the inhibition of the rate of poly U synthesis from UDP by poly A, and showed further, that, at several poly A concentrations, the enzyme was released from the inhibition when enough poly U was synthesized to complex the poly A completely as (A)n - 2(U)n.
I would very much appreciate receiving a reprint of the paper in the PNAS.
Thank you.
Sincerely yours,
Maxine Singer