From the Nobel Foundation:
Seeking more experience with enzymes, he studied for a year in Copenhagen with Herman Kalckar at the Institute of Cytophysiology and for a second year with Arthur Kornberg at Washington University in St. Louis. Both were very productive and enjoyable experiences. In Copenhagen, Wolfgang Joklik and he discovered a new enzyme that created the nucleoside triphospates for nucleic acid assembly and the next year in Kornberg’s lab, he discovered a previously unknown class of biological compounds - acyl adenylates - intermediates in the formation of fatty acyl-CoAs from fatty acids, ATP and CoA. This type of reaction, he discovered, was also central to the way amino acids are activated as amino-acyl adenylates prior to being linked to tRNAs. By then, he was making the slow transition from classical biochemistry to molecular biology and becoming increasingly preoccupied with how genes act and how proteins are made.
After 6 years in St. Louis, he moved to Stanford University’s Medical Center (1959) to help Kornberg set up the new department of biochemistry. In time, his interests shifted from studies with microorganisms to mammalian cells and he spent a year experimenting with Polyoma and SV40 tumor viruses in mammalian cell culture with Renato Dulbecco at the Salk Institute. Soon after he returned to Stanford, he conceived of using SV40 as a means for introducing new genes into mammalian cells much in the way that bacteriophage transduce cellular DNA among infected cells. He and his colleagues succeeded in developing a general way to join two DNAs together in vitro; in this case, a set of three genes responsible for metabolizing galactose in the bacterium E. coli was inserted into the SV40 DNA genome. That work led to the emergence of the recombinant DNA technology thereby providing a major tool for analyzing mammalian gene structure and function and formed the basis for his receiving the 1980 Nobel Prize in Chemistry.
Nobel Laureate, Chemistry Prize, 1980