  Based on the first-principles calculations, we reveal that TlN, a simple
binary compound with Wurtzite structure, is a three-dimensional (3D)
topological insulator (TI) with effectively negative spin-orbit coupling
$\lambda_{eff} < 0$, which makes it distinguished from other TIs by showing
opposite spin-momentum locking effect in its surface states. The sign of
$\lambda_{eff}$ depends on the hybridization between N-$2p$ and Tl-$5d$ states,
and can be tuned from negative to postive by lattice strain or chemical
substitution, which drive the system into a Dirac semimetal with 3D Dirac cones
in its bulk states. Such topological phase transition can be realized by
electronic mechanism without breaking any crystal symmetry.
