Catalyzes cyclization of the linear tetrapyrrole, hydroxymethylbilane, to the macrocyclic uroporphyrinogen III, a precursor of tetrapyrroles such as chlorophyll, heme and phycobilins. Uroporphyrinogen-III synthase, chloroplastic UROS T9J22.21 MALLLLSHCSILSFQPPLSSSSSFHSSHIQSLSKPVFASPSPIRNSISSSVSSSSSSVSSSNSIPQVVVTRERGKNNQIIKALEKNGISSLELPLIQHARGPDFDRLASVLNDKSFDWIIITSPEAGSVFLEAWKTASSPEVQIGVVGAGTARVFEEAMKSADGLLHVAFTPSKATGKVLASELPEKVGKRSSVLYPASLKAGNDIVEGLSKRGFEVVRLNTYTTVPVQSVDTVLLQQALSAPVLSVASPSAVRAWLHLIQNEEQWSNYVACIGETTASAARRLGLKNVYYPEKPGLEGWVESIMEALGAHADSSNPSSRN HEM4_ARATH 321 At2g26540 HEMD Uroporphyrinogen-III cosynthase Hydroxymethylbilane hydrolyase [cyclizing]