A NOVEL INVOLVEMENT OF THE PURG AND PURI PROTEINS IN THIAMINE SYNTHESIS VIA THE ALTERNATIVE PYRIMIDINE BIOSYNTHETIC (APB) PATHWAY IN SALMONELLA-TYPHIMURIUM

Thiamine is thought to be synthesized by two alter-native pathways, on e involving the first four enzymes of the purine pathway and a second that can function independently of the purine pathway. Insertion mutat ions in purG and purI prevent thiamine synthesis through the alternati ve pyrimidine biosynthetic (APE) pathway under aerobic but not anaerob ic growth conditions, In contrast, point mutations in purG and purI ca used one of three distinct phenotypes: Pur(-) Apb(-), Pur(-) Apb(+), o r Pur(+) Apb(-). Analysis of these three mutant classes demonstrated t wo genetically separable functions for PurG and PurI in thiamine synth esis. In addition to their known enzymatic role in de novo purine synt hesis, we propose that PurG and PurI play a novel, possibly nonenzymat ic role in the APE pathway. Suppression analysis of Pur(-) Apb(-) muta nts identified two new genetic loci involved in the APB pathway, apbB and akbD. We show here that mutations in apbB and apbD cause distinct, allele-specific suppression of the thiamine requirement of purG and p urI mutants. Our results suggest that PurG and PurI and one or more bs components of the APB pathway may function as a complex needed for ae robic function of the APD pathway.