Cracking the Non-Ribosomal Code

Following Watson & Crick’s publication of DNA’s double helix structure in 1953, physicist George Gamow founded the “RNA Tie Club” for renowned scientists. A necktie embroidered with a double helix signified membership in this club, which was restricted to twenty regular members (one for each amino acid) as well as four honorary members (one for each nucleotide). Gamow wanted the RNA Tie Club to serve more than a social function; by convening top scientific minds, he hoped to decode the message hidden within DNA by determining how RNA is converted into amino acids. Indeed, Sydney Brenner and Francis Crick struck first one year later by discovering that amino acids are translated from codons (i.e., triplets of nucleotides).

The RNA Tie Club would eventually boast eight Nobel laureates, but scientists from outside of the club would decipher the genetic code. In 1961, Marshall Nirenberg synthesized RNA strands consisting only of uracil (UUUUUUUUUUUU. . .), added ribosomes and amino acids, and produced a peptide consisting only of phenylalanine (PhePhePhePhe. . .). Nirenberg thus concluded that the RNA codon UUU codes for the amino acid phenylalanine. Following Nirenberg’s success, Har Gobind Khorana synthesized the RNA strand UCUCUCUCUCUC. . . and demonstrated that it translates into SerLeuSerLeu. . . Following these insights, the rest of the ribosomal genetic code was rapidly elucidated.

NRP synthetase

Nearly four decades later, Mohamed Marahiel set out to solve the much more challenging puzzle of cracking the non-ribosomal code. Bacteria and fungi produce antibiotics and other non-ribosomal peptides (NRPs) without any reliance on the ribosome and the genetic code. Instead, these organisms manufacture NRPs by employing a giant protein called NRP synthetase:

DNA $\rightarrow$ RNA $\rightarrow$ NRP $\rightarrow$ synthetase $\rightarrow$ NRP

The NRP synthetase that encodes the 10 amino acid-long antibiotic Tyrocidine B1 includes 10 segments called adenylation domains (A-domains); each A-domain is about 500 amino acids long and is responsible for adding a single amino acid to Tyrocidine B1.

A generation earlier, the RNA Tie Club had asked, “How does RNA encode an amino acid?” Now Marahiel set out to answer the far more challenging question, “How does each A-domain encode an amino acid?”