The primary function of the sigma protein is to recognize and bind to the -35 and -10 regions of the gene in a bacterial promoter. The sigma factor is a part of the holoenzyme of the ribonucleic acid (RNA) polymerase enzyme that aids in recognizing the promoter sequence. It is only after the correct positioning of the RNA polymerase any gene could be transcribed.
Apart from the sigma factor, other proteins too tend to behave in the same way in which the sigma factor behaves. These proteins include the TATA binding protein (TBP) and other general transcription factors (GTF).