Abstract In Escherichia coli K 12, arginine modulates the functioning of the arginine transport system. Cells grown in the presence of arginine show a 60 % reduction in the active incorporation of radioactive arginine. This regulation of arginine transport is independent of the regulation of arginine biosynthesis. Previously, a mutant was isolated with a 90 % reduction of arginine transport. The mutation affected also the transport of ornithine and lysine. It was mapped and assigned to a locus named argP at minute 65 of the E. coli linkage map. Genetic studies showed that in argP/argP + merodiploids, the mutated argP allele is dominant. The argP + gene was cloned and sequenced. Analysis of the sequenced gene revealed that it is identical with iciA, an E. coli gene that encodes an inhibitor of chromosomal initiation of replication in vitro. The sequence analysis of the mutated argP gene identified a single mutation that led to the substitution of proline for serine in the C-terminal domain of the ArgP protein. This protein has homology with a large group of prokaryotic regulatory proteins known as the LysR family. Proteins from this family have been shown to function as transcriptional regulators. Here, it is shown that the ArgP protein activates the formation of the ArgK protein, an ATP-binding protein essential for the operation of the arginine transport system. In the presence of l-arginine, ArgP inhibits its own synthesis.