Final answer:
In bacterial RNA polymerase, the transcription bubble is maintained open by the interactions of its core subunits, with the β' subunit playing a key role in binding the DNA template strand.
Step-by-step explanation:
In bacterial RNA polymerase, the structure that helps keep the transcription bubble open is primarily the result of interactions between subunits within the polymerase complex itself. The core enzyme, which is composed of several subunits including two α-subunits, β, and β', maintains the transcription bubble as it synthesizes mRNA. The β' subunit has a critical role in binding the DNA template strand, which assists in maintaining the unwound state of DNA that is necessary for transcription. Moreover, the transcription bubble is a region of locally unwound DNA where transcription of mRNA takes place, and the strong interactions between the RNA polymerase subunits and the DNA strand are essential for keeping it stable during transcription.