Cloning and characterization of a 12-gene cluster from Bacillus subtilis encoding nine enzymes for de novo purine nucleotide synthesis

An approximately 16-kilobase pair region of the Bacillus subtilis chromosome at 55 degrees containing genes for de novo purine nucleotide synthesis (Piggot, P. J., and Hoch, J. A. (1985) Microbiol. Rev. 49, 158-179) was cloned. The nucleotide sequence of over 13 kilobase pairs indicates that this region contains a cluster of 12 genes, 11 of which encode enzymes that catalyze the 10 reactions for de novo purine nucleotide synthesis from 5-phosphoribosyl 1-pyrophosphate to IMP. The genes were identified by complementation of Escherichia coli pur mutants and by sequence comparisons with homologous enzymes. The cluster is likely an operon and is organized into three groups of overlapping genes followed by the last gene: purEKB-purC(orf)QLF-purMNH(J)-purD. Sequence comparisons provide evidence for homology of monofunctional purine nucleotide biosynthetic enzymes from B. subtilis with the corresponding multifunctional enzymes from yeast and Drosophila. Sequence alignment of the phosphoribosylaminoimidazole carboxylase heterodimer from B. subtilis with the monomeric enzyme from Methanobrevibacter smithii indicates an evolutionary relationship between these two enzymes. S1 nuclease analysis was used to map the mRNA 5' and 3' ends and to estimate levels of mRNA. These experiments indicate that synthesis of purine nucleotides is regulated independently by adenine and guanine nucleotides. Adenine nucleotides regulate transcription initiation. Guanine nucleotides regulate transcription by a termination-antitermination mechanism in a 242-nucleotide 5' untranslated mRNA leader region. Groups of overlapping genes, regulated at least in part by transcription termination-antitermination is likely to be a common theme for genetic organization and regulation of biosynthetic genes in this Gram-positive organism.