Altogether , these observations suggested that the residual chiP induction by chitobiose in the ΔchiX background is unlikely to be due to direct activation by ChbR at the chiP promoter .
From the data in Fig. 3 , it is apparent that inactivation of the ChbR repressor completely prevents residual chiP induction by chitobiose in ΔchiX background in E. coli as it does in Salmonella .
Given that the E. coli sequence lacks the ChbR binding site and shows no binding to the purified protein in-vitro , these findings suggest that the ChbR requirement for the residual induction of chiP by chitobiose is independent of ChbR binding to the chiP promoter region .
Final proof that the ChbR binding site at − 180 in Sal-monella is not implicated in chitobiose induction of chiP came from replacing the ChbR binding site in Salmonella with h 11 -- lacZ fusion construct on the chromosome .
Intriguingly , while footprint analysis confirmed such binding , we obtained no evidence for a ChbR involvement in chiP induction other than its indirect role as activator of chbBCARFG transcription .
Furthermore , replacement of the ChbR binding site in the Salmonella chiP promoter region with the sequence did not affect chiP induction by chitobiose to any significant extent .
Activation of chbBCA expression by ChbR is essential to relieve ChiX repression of chiP .