RPB3 is a core subunit of RNA polymerase II (pol II) that, together with the RPB11 subunit, forms the heterodimer considered as a functional counterpart of the bacterial alpha subunit homodimer involved in promoter recognition. We previously employed the yeast two-hybrid system and identified an interaction between RPB3 and the myogenic transcription factor myogenin, demonstrating an involvement of this subunit in muscle differentiation. In this paper we report the interaction between RPB3 and another known transcription factor, ATF4. We found that the intensity of the interaction between RPB3 and ATF4 is similar to the one between RPB3 and myogenin. This interaction involves an RPB3 specific region not homologous to the prokaryotic alpha subunit. We demonstrated that RBP3 is able to enhance ATF4 transactivation, whereas the region of RPB3 (Sud) that contacts ATF4, when used as a dominant negative, markedly inhibits ATF4 transactivation activity. Interestingly, ATF4 protein level, as reported for its partner RPB3, increases during C2C7 cell line muscle differentiation.