On the one hand, this type of organizational structure benefits from having individuals entirely dedicated to one function. In some cases, there can be cost savings and efficiency gains in combining functions in this way. However, this type of organizational structure is prone to conflict.
The alpha subunit (RpoA) initiates RNAP assembly by dimerising to form a platform on which the beta subunits can interact. The alpha subunit consists of a N-terminal domain (NTD) and a C-terminal domain (CTD), connected by a short linker.
The RNA polymerase is composed of a core of proteins, and if this core of proteins also includes a Sigma subunit, the RNA polymerase is considered a holoenzyme and can then initiate transcription from the promotor sequence on the DNA. Source(s): My bum.