The transition zone differs from the rest of the mantle by high positive P and S wave velocity gradients. However, by applying S receiver function technique to recordings of about 50 globally distributed stations, in 7 regions we obtain evidence of negative discontinuity at a depth around 500 km. Four of these regions correspond to the well known hotspots. No other hotspots are sampled by our observations at this depth. Our modeling suggests that the negative discontinuity is hardly an effect of anisotropy of wadsleyite. More likely, it is caused by isotropic S velocity reduction of around 0.2 km/s. Origin of the low velocity can be related to enhanced water content in wadsleyite.