2. If the two foci of the hyperbola $C$ are exactly the two vertices of the ellipse $\Gamma$ : $\frac{x^{2}}{16}+\frac{y^{2}}{9}=1$, and the two vertices of the hyperbola $C$ are exactly the two foci of the ellipse $\Gamma$, then the equation of the hyperbola $C$ is $\qquad$ .