, 2006). Next, the β-Gal activities from WK074 cells expressing either

wild-type His-Irr or mutant His-Irr proteins were compared. The β-Gal activities obtained were normalized to those from WK074 harbouring the pBBR vector (100% β-Gal activity, GDC-0068 no repression of mbfA-lacZ) (Fig. 2a). WK074 cells expressing wild-type His-Irr (pHIRR) had 1.99% β-Gal activity (Fig. 2a). A single mutation in His-Irr proteins at H38, D86, H92, H93 or D105 could repress mbfA-lacZ as effectively as wild-type His-Irr (1.39, 1.04, 0.97, 1.29 and 0.94% β-Gal activity, respectively) (Fig. 2a). A single mutation at H45, H65 or H127 in the protein caused a slight defect in the ability of the protein to repress mbfA-lacZ compared with wild-type His-Irr, as indicated by the increase learn more in β-Gal activities (3.83%, 4.77% and 8.96% β-Gal activity, respectively) (Fig. 2a).

The H94 mutation caused the greatest reduction in the repressor function of His-Irr (17.23% β-Gal activity) as compared with the mutations at the other H residues (Fig. 2a). A double mutation at residues H45 and H65 of His-Irr (corresponding to the second haem-binding site of IrrRl) caused a small defect (H45H65, 11% β-Gal activity). Triple mutation in the HHH motif of His-Irr (H92, H93 and H94) caused a large defect in the repressor function of the protein (HHH, 63% β-Gal activity) but did not completely abolish protein function (Fig. 2b). Based on this, it is likely that amino acid residues outside

of the HHH motif also contribute to the repressor function of His-Irr. The plasmids containing the mutated HHH motif in combination with the mutation of other residues, including H38, H45, H65, D86, D105 or H127, were constructed to produce the mutant His-Irr proteins HHH38, HHH45, HHH65, HHH86, HHH105 and HHH127, respectively. Additional mutations at H45, H65 or H127 together with the HHH motif mutation led to the complete loss of His-Irr function (HHH45, HHH65 and HHH127: 103%, 101% and 99% β-Gal activity, SB-3CT respectively) (Fig. 2b). Although the mutant His-Irr proteins HHH38 and HHH105 both showed an additive effect compared to HHH, the mutant proteins did not lose function completely (76% and 85% β-Gal activity, respectively) (Fig. 2b). Unexpectedly, an additional mutation at D86 could fully reverse the defect caused by the HHH mutation (HHH86, 0.87% β-Gal activity) (Fig. 2b). The experiments were repeated using the plasmid pIRR to express wild-type IrrAt that encodes the native protein without the 6× His tag. As previously described, the results from the mutagenesis of His-Irr (Fig. 2) showed that H45, H65, D86, H94, the HHH motif and H127 influence the function of Irr.