Seryl tRNA synthetase. Coloured molecular graphic of seryl tRNA synthetase, an enzyme that charges tRNA (transfer ribonucleic acid) molecules with specific amino acids in protein synthesis. The two polypeptide chains are shown in colour. (Overlaps appear pink.) Seryl tRNA synthetase is one of the aminoacyl-tRNA synthetases, a group of molecules that play a fundamental role in manufacturing proteins from genes. This particular aminoacyl- tRNA synthetase bears no resemblance to other types previously determined, a finding that presents molecular biologists with an outstanding puzzle in the evolution of the genetic mechanism.