The sigma factor (s) causes RNA polymerase to bind to the correct sites on DNA to initiate transcription (i.e. promoters). s destabilizes the complex between core polymerase and non-promoter DNA and decreases the amount of time it is bound. It enhances the affinity and increases the amount of time that holoenzyme (a2bb's) is bound to promoter, i.e. it facilitates a random search for promoters.