Nirenberg and Matthaei experiment

Twenty tubes, one answer

At approximately 3:00 a.m. on 27 May 1961, the NIH postdoctoral fellow J. Heinrich Matthaei added synthetic RNA made entirely of uracil nucleotides — poly-U — to an E. coli cell-free protein-synthesis system.

The assay ran across 20 tubes, each with a different radioactively labelled amino acid. Only the phenylalanine tube produced the striking radioactive signal, showing that poly-U directed phenylalanine incorporation into a polypeptide.

What the paper actually claimed

Marshall W. Nirenberg had built the cell-free system with Matthaei, and the two interpreted and published the result jointly. The published paper concluded that polyuridylic acid carried information for making a protein resembling poly-L-phenylalanine and could act as a synthetic messenger-RNA template.

It was more cautious than later retellings. It concluded only that one or more uridylic-acid residues coded for phenylalanine, and said explicitly that whether the code was singlet, triplet or another length had not been determined.

Why 15 May is the wrong timestamp

A 2025 NIH obituary says the experimental series began on 15 May; the American Chemical Society's historical reconstruction dates the decisive phenylalanine assay to 3:00 a.m. on 27 May. The earlier date marks a beginning, not the result.

Matthaei physically performed the overnight assay, but describing him as working “alone” misleads if it implies sole intellectual authorship of a collaborative program.

Nirenberg presented the finding in Moscow in August 1961 to an audience of about 35 people; after Francis Crick helped arrange a repeat presentation, the larger audience gave him a standing ovation.