5, lane 12). An extensive mutagenesis of the E. coli ArgR was performed in order to determine its precise role in cer site-specific recombination. The ArgR protein binds DNA at ARG boxes localized in promoter regions of several genes of the arginine regulon and at the cer site, which

contains half an ARG box. It is quite likely that the DNA-binding activity of this protein is an important contributor to its role as an accessory factor in cer recombination by bringing two cer sites together. However, ArgR by itself cannot support cer recombination in the presence of the XerCD recombinases; PepA is also required for this reaction. Alén et al. (1997) have demonstrated that PepA and ArgR interact directly with cer, forming a complex in which the accessory sequences of two cer sites are interwrapped approximately three times in a right-handed fashion. Using pentapeptide Selleckchem ABT888 scanning mutagenesis, we isolated a series of ArgR mutants that showed an approximate 90% reduction in cer recombination, but were still able to repress an argA∷lacZ fusion effectively in vivo (Figs 1 and 2). The mutant proteins also displayed

sequence-specific DNA-binding activity (Fig. 3). All of Obeticholic Acid mouse the insertions mapped to the same amino acid, between residues 149 and 150 of ArgR (ArgR5aa). This region corresponds to the C-terminal region of ArgR, at the end of the α6-helix (Fig. 4). In order to show that the observed phenotype was due to the disruption of ArgR and was not caused by the additional five amino acids residues, we constructed an ArgR mutant that was truncated at this region. This protein lacks residues 150–156 (ArgR149), Anidulafungin (LY303366) but displays the same properties as ArgR5aa, namely a significant reduction

in cer site-specific recombination in vivo (Fig. 1b), and the ability to bind to DNA at near wild-type levels in vivo (Fig. 2) and in vitro (Fig. 3). Moreover, we were able to detect the same level of DNA retardation as the wild-type protein, which suggests that both ArgR149 and ArgR5aa bind to DNA as hexamers (Fig. 3). In addition, crosslinking studies have shown that wild-type and mutant proteins are capable of forming higher-order structures in solution, although ArgR149 does not appear to form hexamers as efficiently as either wild-type ArgR or ArgR5aa under the crosslinking conditions used (Fig. 5). It is possible that the small C-terminal deletion in ArgR149 prevents this protein from forming a stable hexameric structure. Similar results have been observed with the α A-crystallin protein, where deletions of the terminal 11 amino acids from the C-terminus significantly decreased the oligomeric size of the protein (Thampi & Abraham, 2003). Despite this, ArgR149 can still bind DNA effectively both in vivo and in vitro; the addition of DNA and l-arginine may allow this mutant to form more stable hexamers under these conditions.