The properties of a wave equation for a six-component wave function of a photon are re-analyzed. It is shown that the wave equation presents all the properties required by quantum mechanics, except for the ones that are linked with the definition of the position operator. The situation is contrasted with the three-component formulation based on the Riemann-Silberstein wave function. The inconsistency of the latter with the principles of quantum mechanics is shown to arise from the usual interpretation of the wave function. Finally, the Lorentz invariance of the six-component wave equation is demonstrated explicitly for Lorentz boosts and space inversion.