Role of ArgP (IciA) in Lysine-Mediated Repression in Escherichia coli

Initially identified as an inhibitor of oriC-initiated DNA replication in vitro, the ArgP or IciA protein of Escherichia coli has subsequently been described as a nucleoid-associated protein and also as a transcriptional regulator of genes involved in DNA replication (dnaA and nrdA) and amino acid metabolism (argO, dapB, and gdhA [the last in Klebsiella pneumoniae]). ArgP mediates lysine (Lys) repression of argO, dapB, and gdhA in vivo, for which two alternative mechanisms have been identified: at the dapB and gdhA regulatory regions, ArgP binding is reduced upon the addition of Lys, whereas at argO, RNA polymerase is trapped at the step of promoter clearance by Lys-bound ArgP. In this study, we have examined promoter-lac fusions in strains that were argP(+) or ΔargP or that were carrying dominant argP mutations in order to identify several new genes that are ArgP-regulated in vivo, including lysP, lysC, lysA, dapD, and asd (in addition to argO, dapB, and gdhA). All were repressed upon Lys supplementation, and in vitro studies demonstrated that ArgP binds to the corresponding regulatory regions in a Lys-sensitive manner (with the exception of argO, whose binding to ArgP was Lys insensitive). Neither dnaA nor nrdA was ArgP regulated in vivo, although their regulatory regions exhibited low-affinity binding to ArgP. Our results suggest that ArgP is a transcriptional regulator for Lys repression of genes in E. coli but that it is noncanonical in that it also exhibits low-affinity binding, without apparent direct regulatory effect, to a number of additional sites in the genome.