Transcription factor that binds to a symmetrical DNA sequence (E-boxes) (5'-CACGTG-3') that is found in a variety of viral and cellular promoters. Regulates the expression of the surfactant protein-A (SP-A) gene. Efficient DNA binding requires dimerization with another bHLH protein. Binds DNA as a homodimer or a heterodimer (USF1/USF2).