In this study, we investigated the functional role of early growth response-1 (Egr1 gene) in the regulation of radiation-induced clonogenic inhibition and apoptosis in p53 wild-type and mutant prostate cancer cells 22Rv1 and DU145, respectively. 22Rv1 cells were more sensitive to irradiation compared with DU145 cells, and the sensitivity was enhanced by overexpression of EGR-1 in both cells. Dominant-negative EGR-1 mutant (dnEGR-1) or repressor of EGR-1, NGFIA binding protein 1 (NAB1), increased radioresistance of these cells. Significant activation of caspases 3 and 9 and Bcl2-associated X (Bax) with increased poly(ADP-ribose) polymerase (PARP) cleavage and cytochrome c release was observed in radiation-exposed EGR-1 overexpressing cells. Gel shift analysis and chloramphenicol acetyl transferase (CAT) reporter assays indicate that EGR-1 transactivates the promoter of the Bax gene. Interaction of EGR-1 and Yes kinase-associated protein 1 (YAP-1) through the WW domain of YAP-1 enhances the transcriptional activity of EGR-1 on the Bax promoter as shown by chromatin immunoprecipitation and reporter assays. Irradiation of PC3 cell xenografts that were treated with adenoviral EGR-1 showed significant regression in tumor volume. These findings establish the radiation-induced pro-apoptotic action of EGR-1, in a p53-independent manner, by directly transactivating Bax, and prove that alters the B-cell CLL/lymphoma 2 (Bcl-2)/Bax ratio as one of the mechanisms resulting in significant activation of caspases, leading to cell death through the novel interaction of EGR-1 with YAP-1.