  Ultraviolet-photoemission (UPS) measurements and supporting specific-heat,
thermal-expansion, resistivity and magnetic-moment measurements are reported
for the magnetic shape-memory alloy Ni$_2$MnGa over the temperature range $100K
< T < 250K$. All measurements detect clear signatures of the premartensitic
transition ($T_\mathrm{PM}\sim 247K$) and the martensitic transition
($T_\mathrm{M} \sim 196K$). Temperature-dependent UPS shows a dramatic
depletion of states (pseudogap) at $T_\mathrm{PM}$ located 0.3eV below the
Fermi energy. First-principles electronic structure calculations show that the
peak observed at 0.3eV in the UPS spectra for $T > T_\mathrm{PM}$ is due to the
Ni-d minority-spin electrons. Below $T_\mathrm{M}$ this peak disappears,
resulting in an enhanced density of states at energies around 0.8eV. This
enhancement reflects Ni-d and Mn-d electronic contributions to the
majority-spin density of states and is accompanied by significant
reconstruction of the Fermi surface.
