Utilization of orotate as a pyrimidine source by Salmonella typhimurium and Escherichia coli requires the dicarboxylate transport protein encoded by dctA.

Abstract

Mutants deficient in orotate utilization (initially termed out mutants) were isolated by selection for resistance to 5-fluoroorotate (FOA), and the mutations of 12 independently obtained isolates were found to map at 79 to 80 min on the Salmonella typhimurium chromosome. A gene complementing the mutations was cloned and sequenced and found to possess extensive sequence identity to characterized genes for C4-dicarboxylate transport (dctA) in Rhizobium species and to the sequence inferred to be the dctA gene of Escherichia coli. The mutants were unable to utilize succinate, malate, or fumarate as sole carbon source, an expected phenotype of dctA mutants, and introduction of the cloned DNA resulted in restoration of both C4-dicarboxylate and orotate utilization. Further, succinate was found to compete with orotate for entry into the cell. The S. typhimurium dctA gene encodes a highly hydrophobic polypeptide of 45.4 kDa, and the polypeptide was found to be enriched in the membrane fraction of minicells harboring a dctA+ plasmid. The DNA immediately upstream of the deduced -35 region contains a putative cyclic AMP-cyclic AMP receptor protein complex binding site, thus affording an explanation for the more effective utilization of orotate with glycerol than with glucose as carbon source. The E. coli dctA gene was cloned from a lambda vector and shown to complement C4-dicarboxylate and orotate utilization in FOA-resistant mutants of both E. coli and S. typhimurium. The accumulated results demonstrate that the dctA gene product, in addition to transporting C4-dicarboxylates, mediates the transport of orotate, a cyclic monocarboxylate.