The nucleotide sequence of a 1634 bp DNA fragment from the photosynthetic purple sulfur bacterium Allochromatium vinosum contains one complete and two partial open reading frames. Sequence comparisons to genes from other organisms suggest that this A. vinosum DNA fragment contains, starting from the 5 end, the following: (1) 234 bp at the 3 end of the A. vinosum purH gene, coding for 78 amino acids at the C-terminus of the bi-functional 5-phosphoribosyl-5-aminoimidazole-4-carboxamide formyltransferase/IMP cyclohydrolase (EC 2.1.2.3), an enzyme involved in de novo purine biosynthesis; (2) 777 bp of the A. vinosum lpxA gene, coding for all 259 amino acids of the UDP-N-acetylglucosamine-O-acyltransferase, an enzyme involved in lipid A biosynthesis; and (3) 567 bp at the 5 end of the A. vinosum purD gene, coding for 189 amino acids at the N-terminus of 5-phosphoribosyl glycinamide synthetase (EC 6.3.4.13), a second enzyme involved in de novo purine biosynthesis. The presence of a gene coding for an enzyme involved in lipid A biosynthesis between two genes coding for enzymes of the de novo purine biosynthesis pathway represents a unique arrangement of these genes.