sentence: Interestingly , we do not detect a comparable signal when the sole histidine residue of NPr is mutated to alanine ( Fig. S3 ) , which could indicate weak cross-reactivity of the antibodies with unphosphorylated histidine on NPr.From the in vitro results presented above , we conclude that SixA directly dephosphorylates NPr-P , and the reaction proceeds by phosphoryl transfer from π - pHis on NPr to τ - pHis on SixA .