Glutamate-1-semialdehyde aminotransferase (GSA-AT), also named glutamate-l-semialdehyde aminomutase (GSAM), a pyridoxamine-5'-phosphate (PMP)/pyridoxal-5'-phosphate (PLP) dependent enzyme, catalyses the transamination of the substrate glutamate-l-semialdehyde (GSA) to the product 5-Aminolevulinic acid (ALA) by an unusual intramolecular exchange of amino and oxo groups within the catalytic intermediate 4,5-diaminovalerate (DAVA). This paper presents the crystal structure of GSA-AT from Bacillus subtilis (GSA-AT(Bsu)) in its PMP-bound form at 2.3 angstrom resolution. The structure was determined by molecular replacement using the Synechococcus GSAM (GSAM(syn)) structure as a search model. Unlike the previous reported GSAM/GSA-AT structures, GSA-AT(Bsu) is a symmetric homodimer in the PMP-bound form, which shows the structural symmetry at the gating loop region with open state, as well as identical cofactor (PMP) binding in each monomer. This observation of PMP in combination with an "open" lid supports one characteristic feature for this enzyme, as the catalyzed reaction is believed to be initiated by PMP. Furthermore, the symmetry of GSA-ATBsu structure challenges the previously proposed negative cooperativity between monomers of this enzyme. (C) 2010 Elsevier Inc. All rights reserved.