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 (PubMed:10722597). 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 (PubMed:20207758). Binds 1 Mg(2+) ion per subunit. Interacts (when phosphorylated) with FliM. Phosphorylated by CheAY (PubMed:16207913). Dephosphorylated (inactivated) by CheZ (PubMed:16207913, PubMed:20497335).