Chemotactic response regulator protein that modulates the rotation direction of bacterial flagellar motors. Plays an important role in the colonization and infection of Helicobacter pylori. Upon phosphorylation by CheA, interacts with the flagellar motor protein FliM to cause clockwise flagellar rotation and bacterial reversals, as opposed to straight swimming when CheY1 is not phosphorylated. Binds 1 Mg(2+) ion per subunit. Interacts (when phosphorylated) with FliM. Phosphorylated by CheAY. Dephosphorylated (inactivated) by CheZ.