On the other hand, phenotypical analysis with an AtoS mutant showed reduced motility, sodium chloride sensitivity, and increased susceptibility to membrane-acting agents and an aminoglycoside antibiotic [9]. AtoSC is involved in the flagellar regulon, controlling the motility and chemotaxis responses [10]. Through atoDAEB operon regulation, AtoC modules positively the biosynthesis and the intracellular distribution of poly-(R)-3-hydroxybutyrate (cPHB), a biopolymer with many physiological roles [11]. The products of the catabolism of short-chain fatty acids appear to be the substrates in the biosynthesis of this polymer [11] By an unknown mechanism, histamine and extracellular Ca²⁺ appear to be signals that modulate AtoS/AtoC-dependent cPHB production [12, 13]. AtoC is the response regulator in the two-component regulatory system AtoC/AtoS [1]. Autophosphorylation of AtoS appears to be stimulated by acetoacetate or one of its metabolic products [11]. Then, the phosphate group is transferred from AtoS to the D55 residue of AtoC to activate this protein [1, 14]. It has also been suggested that the D55 residue could transfer the phosphate group to the H73 residue of AtoC [14]. AtoC belongs to the NtrC-NifA family of σ⁵⁴-RNA polymerase transcriptional activators [14] and, as for a number of members of this family, AtoC contains three domains: the N-terminal receiver domain to be phosphorylated, the central domain involved in ATP hydrolysis and interaction with σ⁵⁴, and the DNA-binding C-terminal domain that contains a helix-turn-helix motif [14]. It was suggested that AtoC oligomerizes, but the stoichiometry is unknown [5]. This protein recognizes and binds a sequence of 20 bp in DNA [4]. In the atoDAEB regulatory region there are two of these sites arranged as inverted repeats separated by 1 bp [4]. However, a BLAST analysis showed that although there are several regions with significant homology to the 20-bp AtoC-binding site, they are not arranged as palindromes [4]. AtoC-dependent atoDAEB transcription requires the presence and binding of IHF [4]. Proteins with antizyme activity have been found in several organisms, from plants to eubacteria [5]. A BLAST analysis showed AtoC is homologous in proteobacteria, enterobacteria, eubacteria, and spirochetes [5]. The two-component regulatory system AtoC/AtoS system has been used to create a biosensor for detection of acetoacetate for industrial, medical, and environmental applications [15]. atoC, encoding the response regulator, and atoS, encoding the sensor kinase, are located in the genome upstream of the atoDAEB operon [3]. The transcription of the gene atoC is induced by polyamines [16] and histamine [12]; microarray analysis shows that OmpR may regulate the transcription of this gene [8]. On the other hand, it does not appear to be autoregulated [2]. AtoC: acetoacetate [7] Reviews: [5, 13]