The invention pertains to isolated phosphopantetheinyl transferases, such as the E. coli acyl carrier protein synthase, which transfer a phosphopantetheinyl group onto a substrate. The enzyme can be purified from a natural source, produced recombinantly, or synthetically. Accordingly, the invention provides compositions and kits including phosphopantetheinyl transferases and host cells expressing phosphopantetheinyl transferases. The invention also provides nucleic acids encoding phosphopantetheinyl transferases and vectors comprising such nucleic acids. The invention further provides methods for phosphopantetheinylating a substrate in vitro or in vivo and methods for producing antibiotics in vitro or in vivo.