ESCHERICHIA-COLI AMINODEOXYCHORISMATE SYNTHASE - ANALYSIS OF PABB MUTATIONS AFFECTING CATALYSIS AND SUBUNIT ASSOCIATION

p-Aminobenzoic acid (PABA), an essential component of the vitamin foli c acid, is derived from the aromatic branch-point precursor chorismate in two steps. 4-Amino-4-deoxychorismate (ADC) synthase converts chori smate and glutamine to ADC and glutamate, and is composed of two subun its, PabA and PabB. While various experiments have suggested that PabA and PabB act as a complex, attempts to isolate the intact complex hav e failed. We report here the first successful copurification of PabA a nd PabB by gel filtration chromatography. The association of PabA and PabB is greatly enhanced by the presence of 5 mM glutamine, and by pre incubation at 37 degrees C. Conversely, the association is greatly red uced at cold temperatures. We also report the isolation and characteri zation of both chemically induced and site-directed mutations in PabB. Mutated PabB enzymes fall into three categories according to their pr operties: deficiency of chorismate amination coupled with failure to a ssociate with PabA, deficiency of chorismate amination coupled with re tention of PabA association, and competency of chorismate amination wi th failure of PabA association.