Thiamine is thought to be synthesized by two alternative pathways, one involving the first four enzymes of the purine pathway and a second that can function independently of the purine pathway. Insertion mutations in purG and purI prevent thiamine synthesis through the alternative pyrimidine biosynthetic (APB) pathway under aerobic but not anaerobic growth conditions. In contrast, point mutations in purG and purI caused one of three distinct phenotypes: Pur- Apb-, Pur- Apb+, or Pur+ Apb-. Analysis of these three mutant classes demonstrated two genetically separable functions for PurG and PurI in thiamine synthesis. In addition to their known enzymatic role in de novo purine synthesis, we propose that PurG and PurI play a novel, possibly nonenzymatic role in the APB pathway. Suppression analysis of Pur- Apb- mutants identified two new genetic loci involved in the APB pathway, apbB and apbD). We show here that mutations in apbB and apbD cause distinct, allele-specific suppression of the thiamine requirement of purG and purI mutants. Our results suggest that PurG and PurI and one or more components of the APB pathway may function as a complex needed for aerobic function of the APB pathway.