Tetrapolymerization of the monopyrrole PBG into the hydroxymethylbilane pre-uroporphyrinogen in several discrete steps. H2O + 4 porphobilinogen = hydroxymethylbilane + 4 NH4(+) Binds 1 dipyrromethane group covalently. Porphyrin-containing compound metabolism; protoporphyrin-IX biosynthesis; coproporphyrinogen-III from 5-aminolevulinate: step 2/4. The porphobilinogen subunits are added to the dipyrromethan group. Belongs to the HMBS family.