APBA, A NEW GENETIC-LOCUS INVOLVED IN THIAMINE BIOSYNTHESIS IN SALMONELLA-TYPHIMURIUM

In Salmonella typhimurium, the synthesis of the pyrimidine moiety of t hiamine can occur by utilization of the first five steps in de novo pu rine biosynthesis or independently of the pur genes through the altern ative pyrimidine biosynthetic, or APB, pathway (D. M. Downs, J. Bacter iol. 174:1515-1521, 1992). We have isolated the first mutations defect ive in the APB pathway. These mutations define the apbA locus and map at 10.5 min on the S. typhimurium chromosome. We have cloned and seque nced the apbA gene and found it to encode a 32-KDa polypeptide whose s equence predicts an NAD/flavin adenine dinucleotide-binding pocket in the protein. The phenotypes of apbA mutants suggest that, under some c onditions, the APB pathway is the sole source of the pyrimidine moiety of thiamine in wild-type S. typhimurium, and furthermore, the pur gen etic background of the strain influences whether this pathway can func tion under aerobic and/or anaerobic growth conditions.