Catalyzes two reactions in de novo purine nucleotide biosynthesis. Catalyzes the breakdown of 5-aminoimidazole- (N-succinylocarboxamide) ribotide (SAICAR or 2-[5-amino-1-(5-phospho-beta-D-ribosyl)imidazole-4-carboxamido]succinate) to 5-aminoimidazole-4-carboxamide ribotide (AICAR or 5-amino-1-(5-phospho-beta-D-ribosyl)imidazole-4-carboxamide) and fumarate, and of adenylosuccinate (ADS or N(6)-(1,2-dicarboxyethyl)-AMP) to adenosine monophosphate (AMP) and fumarate. (Microbial infection) Catalyzes the conversion of 2-amino-2'-deoxyadenylo-succinate to dZMP and fumarate, when the bacterium is infected by a phage that produces the substrate of this reaction, a step in the synthesis of dZTP (2-amino-2'-deoxyadenosine 5'-triphosphate), which is a nucleotide then used by the phage as a DNA polymerase substrate. N(6)-(1,2-dicarboxyethyl)-AMP = AMP + fumarate (2S)-2-[5-amino-1-(5-phospho-beta-D-ribosyl)imidazole-4-carboxamido]succinate = 5-amino-1-(5-phospho-beta-D-ribosyl)imidazole-4-carboxamide + fumarate (2S)-2-amino-2'-deoxyadenylo-succinate = dZMP + fumarate Purine metabolism; AMP biosynthesis via de novo pathway; AMP from IMP: step 2/2. Purine metabolism; IMP biosynthesis via de novo pathway; 5-amino-1-(5-phospho-D-ribosyl)imidazole-4-carboxamide from 5-amino-1-(5-phospho-D-ribosyl)imidazole-4-carboxylate: step 2/2. Purine metabolism. Homotetramer. A reduction in the expression of purB leads to a reduction in the efficiency of infection by a Vibrio phage. Belongs to the lyase 1 family. Adenylosuccinate lyase subfamily.