5-Aminolevulinate synthase (EC 2.3.1.37) is the first enzyme in the heme biosynthetic pathway of animals, fungi and some bacteria. It functions as a homodimer and requires pyridoxal 5'-phosphate as an essential cofactor. In mouse erythroid 5-aminolevulinate synthase, lysine 313 has been identified as the residue involved in the Schiff base linkage with pyridoxal 5'-phosphate [Ferreira, G. C., et al. (1993) Protein Sci. 2, 1959-1965], while arginine 149, a conserved residue among all known 5-aminolevulinate synthase sequences, is essential for function [Gong & Ferreira (1995) Biochemistry 34, 1678-1685]. To determine whether each subunit contains an independent active site (i.e., intrasubunit arrangement) or whether the active site resides at the subunit interface (i.e., intersubunit arrangement), in vivo complementation studies were used to generate heterodimers from site-directed, catalytically inactive mouse 5-aminolevulinate synthase mutants. When R149A and K313A mutants were co-expressed in a hem A- Escherichia coli strain, which can only grow in the presence of 5-aminolevulinate or when it is transformed with an active 5-aminolevulinate synthase expression plasmid, the hem A- E. coli strain acquired heme prototrophy. The purified K313A/R149A heterodimer mixture exhibited K(m) values for the substrates similar to those of the wild-type enzyme and approximately 26% of the wild-type enzyme activity which is in agreement with the expected 25% value for the K313A/R149A coexpression system. In addition, DNA sequencing of four Saccharomyces cerevisiae 5-aminolevulinate synthase mutants, which lack ALAS activity but exhibit enzymatic complementation, revealed that mutant G101 with mutations N157Y and N162S can complement mutant G220 with mutation T452R, and mutant G205 with mutation C145R can complement mutant Ole3 with mutation G344C. Taken together, these results provide conclusive evidence that the 5-aminolevulinate synthase active site is located at the subunit interface and contains catalytically essential residues from the two subunits.