The complete Bacillus subtilis genome contains four genes (proG, proH, proI, and comER) with the potential to encode Delta(1)-pyrroline-5-carboxylate reductase, a proline biosynthetic enzyme. Simultaneous defects in three of these genes (proG, proH, and proI) were required to confer proline auxotrophy, indicating that the products of these genes are mostly interchangeable with respect to the last step in proline biosynthesis.