Explain the role of sigma factors in RNA synthesis.

A sigma factor is one of five subunits within an RNA polymerase involved exclusively in transcription of DNA to RNA. It identifies and binds to a promoter (initiation) site, notably the highly conservative -10 and -35 sites, which creates an RNA polymerase-DNA template complex that facilitates transcription initiation. A sigma factor then dissociates from the template after a short stretch of RNA is synthesized. They are also subject to regulation by anti-sigma factor proteins that temporarily inactivate a sigma factor and thus halt RNA synthesis.