Catalyzes an early step in the biosynthesis of tetrapyrroles. Binds two molecules of 5-aminolevulinate per subunit, each at a distinct site, and catalyzes their condensation to form porphobilinogen (By similarity). 2 5-aminolevulinate = H(+) + 2 H2O + porphobilinogen Binds 2 magnesium ions per monomer. The first magnesium ion is required for catalysis. The second functions as allosteric activator. Porphyrin-containing compound metabolism; protoporphyrin-IX biosynthesis; coproporphyrinogen-III from 5-aminolevulinate: step 1/4. Porphyrin-containing compound metabolism; chlorophyll biosynthesis. Homooctamer. Highly expressed in cotyledons during dark-to-light transition. Up-regulated by the transcription factors FAR1 and FHY3. Embryo lethal when homozygous. Impaired plant growth and development. Belongs to the ALAD family.