RNA polymerase (RNAP) is an extensively studied multisubunit enzyme required for transcription of DNA into RNA, yet the δ subunit of RNAP remains an enigmatic protein whose physiological roles have not been fully elucidated. Here, we identify a novel, so far unrecognized function of δ from Bacillus subtilis. We demonstrate that δ affects the regulation of RNAP by the concentration of the initiating nucleoside triphosphate ([iNTP]), an important mechanism crucial for rapid changes in gene expression in response to environmental changes. Consequently, we demonstrate that δ is essential for cell survival when facing a competing strain in a changing environment. Hence, although δ is not essential per se, it is vital for the cell's ability to rapidly adapt and survive in nature. Finally, we show that two other proteins, GreA and YdeB, previously implicated to affect regulation of RNAP by [iNTP] in other organisms, do not have this function in B. subtilis.