Catalyzes the dehydration and aromatization of 5-amino-5-deoxy-3-dehydroshikimate (aminoDHS) to 3-amino-5-hydroxybenzoate (AHBA), a compound that then serves as the starter unit for the assembly of a polyketide during the biosynthesis of rifamycin B and other ansamycin antibiotics. Cannot utilize 5-deoxy-5-amino-3-dehydroquinate (aminoDHQ), 5-deoxy-5-aminoshikimate (aminoSA), quinate, 3-dehydroquinate, or 3-dehydroshikimate (DHS) as substrate. In a complex with RifL, RifK may have a second function in the AHBA pathway, acting as a transaminase introducing the nitrogen into the first pathway intermediate, UDP-3-keto-D-glucose, to give UDP-kanosamine. Appears to use glutamine as the nitrogen donor; NH(4)(+) or asparagine are 30% less effective as nitrogen donors and neither glutamate nor aspartate show activity. 5-deoxy-5-amino-3-dehydroshikimate = 3-amino-5-hydroxybenzoate + H(+) + H2O L-glutamine + UDP-3-oxo-alpha-D-glucose = 2-oxoglutaramate + UDP-alpha-D-kanosamine Binds 1 pyridoxal phosphate per subunit. AHBA synthase activity is activated by 3-deoxy-D-arabinoheptulosonic acid 7-phosphate (DAHP), an intermediate in the shikimate pathway, and is irreversibly inhibited by gabaculine (5-amino-1,3-cyclohexadiene-1-carboxylate). kcat is 6.82 sec(-1) for AHBA synthase activity. Optimum pH is 7.5 for AHBA synthase activity. Retains its activity over a broad range of pH. Over 84% of the maximum activity of AHBA synthase is maintained over a pH range from 7.0 to 9.0. Optimum temperature is 33 degrees Celsius for AHBA synthase activity. Retains its activity over a broad range of temperature. Over 84% of the maximum activity of AHBA synthase is maintained over a temperature range from 28 to 50 degrees Celsius. Antibiotic biosynthesis; rifamycin B biosynthesis. Homodimer (PubMed:9497318). Can interact with RifL (PubMed:21081954). Inactivation of the gene encoding AHBA synthase results in loss of rifamycin formation; production of the antibiotic is restored when the mutant is supplemented with AHBA. Cells lacking this gene do not accumulate aminoDHS, aminoDHQ, or their spontaneous aromatization product, protocatechuate. Mechanistic studies show that the enzyme-bound pyridoxal phosphate forms a Schiff's base with the amino group of 5-amino-5-deoxy-3-dehydroshikimate and catalyzes both an alpha,beta-dehydration and a stereospecific 1,4-enolization of the substrate. Belongs to the degT/dnrJ/eryC1 family.