However, recent studies showed that the operator for ArgP consists of two subsites, each one being most likely contacted by two ArgP subunits, and there is sequence specificity of ArgP binding, with a half-site consensus sequence, 5-CTTAT-, involved in effector-free ArgP binding [5] On the other hand, the alternative binding site (ABS'; T-N₁₁-A from 47 to 37) in the argO operator is preferentially bound in the presence of arginine and it is crucial for transcription activation [5] ArgP induces bending on argO and lysP operators with angles of approximately 60 and 56 , respectively, and without significant differences in the absence versus presence of arginine or lysine [5] When this protein activates genes involved in replication, it appears to bind to their regulatory regions without a coeffector [9, 14]. However, the activity of ArgP depends on arginine and lysine to regulate genes involved in arginine metabolism. These amino acids, which bind to ArgP, do not seem to help the protein to form dimers or to bind to DNA, but they contribute to recruitment of the RNA polymerase and in forming the open complex. However, only the ArgP-arginine complex can produce transcripts, whereas ArgP-lysine appears to inhibit transcription [2, 4]. Lysine greatly reduces ArgP-mediated activation of the ArgP regulon and counteracts the positive effect of arginine on argO transcription. Effector-free ArgP is the active form of the regulator for all regulon members, except activation of argO, which is strictly arginine dependent [5] ArgP is a dimer with an estimated concentration of 100 to 400 molecules per cell (that is, around 200 to 800 nM monomers) [17, 20] ArgP, which belongs to the LysR family, has a helix-turn-helix motif located close to the N terminus. Members of the LysR family that show considerable homology to ArgP include AmpR from Citrobacter freundii, MetR from Salmonella enterica serotype Typhimurium, and NhaR from Pseudomonas putida [15]. The expression of argP is induced by PhoB under phosphate starvation [21]. Contrary to the suggestion made by [8], [4] suggested that argP is not autoregulated. Activation and repression by ArgP are differentially controlled by protein-DNA contacts [5] ArgP is also called IciA ( for "inhibitor of chromosome initiation") [15].