sigma factors are subunits of RNA polymerase. they specifize the action of RNAP, depending on what sigma factor is in complex with RNAP will dictate what genes are preferentially transcripted. RNAP can of course function on its own without the action of Sigma factors but it is highly innefective and non-specific - [Burgess, R.R. (2013). Sigma Factors. _Elsevier eBooks_, pp.432–434. doi:https://doi.org/10.1016/b978-0-12-374984-0.01422-4.](https://www.sciencedirect.com/science/article/abs/pii/B9780123749840014224)