Plates were incubated at 28 °C for 48 h. Each strain was tested in duplicate, and the experiment was repeated a minimum of two Antiinfection Compound Library screening times to ensure the reproducibility of the results. The plasmid pHIRR containing the full length, wild-type irr gene fused to 6× his at the N-terminus was constructed to produce a wild-type recombinant N-terminal 6× His-tagged Irr fusion protein (wild-type His-Irr). The 6× His tag allows IrrAt recombinant protein levels to be monitored. The amino acid residues important for the function of IrrAt were assessed by site-directed mutagenesis of residues
H38, H45, H65, D86, H92, H93, H94, D105 and H127, either individually or in combination (Table 1). These nine amino acid residues of IrrAt were selected for mutagenesis because they correspond to metal-binding or haem-binding sites of proteins in the Fur family (Rudolph et al., 2006a). A comparison of the amino acid sequences of Fur proteins from P. aeruginosa and H. pylori and the Irr proteins IrrBj, IrrRl and IrrAt is shown in Fig. 1. Western blot analysis using an anti-RGS-His monoclonal antibody was performed to check the expression of the 6× His-tagged proteins. A single band with the expected this website size of the 6× His-tagged Irr fusion protein was detected.
The results confirmed that the wild-type His-Irr and all of the mutant His-Irr proteins were successfully produced in A. tumefaciens (Fig. S1). Interestingly, mutations in the C-terminal region at residue D105 or H127 affected the electrophoretic mobility, resulting in slightly faster migration of the mutant proteins than the wild-type His-Irr protein (Fig. S1). IrrAt is a repressor of mbfA, and the irr mutant strain (WK074) has constitutively high expression of mbfA (Ruangkiattikul et al., 2012). The mbfA promoter-lacZ transcriptional fusion was used to assess the repressive activity of
the mutant His-Irr proteins. Expression of mbfA-lacZ from the plasmid pPNLZ01 in wild-type NTL4 cells and irr mutant WK074 cells grown in minimal Leukocyte receptor tyrosine kinase AB medium was determined using a β-galactosidase (β-Gal) activity assay. The β-Gal activities obtained from the NTL4 and WK074 cells expressing the plasmid vector pBBR1MCS-4 (pBBR) were approximately 3.9 ± 0.6 and 14.0 ± 3.9 U mg protein−1, respectively. WK074 cells harbouring the plasmid pHIRR had low levels of β-Gal activity of approximately 0.3 ± 0.1 U mg protein−1, indicating that the wild-type His-Irr protein was functional and able to repress the expression of mbfA-lacZ. The level of β-Gal activity in the complemented strain (WK074 harbouring pHIRR) was much lower than that in the wild-type strain (NTL4 harbouring pBBR). This difference was a result of high expression of wild-type His-Irr from the expression vector causing strong repression of mbfA-lacZ. In contrast, high expression of mbfA-lacZ in WK074 cells could not be reversed by expression of the His-Mur negative control (pHMUR) (14.4 ± 1.9 U mg protein−1).