Grunberg-Manago studied biochemistry and, in 1955, while working in the lab of Spanish-America biochemist Severo Ochoa, she discovered the first nucleic-acid-synthesizing enzyme. Initially, everyone thought the new enzyme was an RNA polymerase used by "E. coli" cells to make long chains of RNA from separate nucleotides. But although the new enzyme could link a few nucleotides together, the reaction was highly reversible and it later became clear that the enzyme, polynucleotide phosphorylase, usually catalyzes the "breakdown" of RNA, not its synthesis.