according to information from http://www.rothamsted.ac.uk/notebook/courses/guide/trans.htm " if the RNA polymerase attaches to a special sequence called a promoter, an additional small protein, the factor sigma, will also attach to the polymerase and lock it on the DNA. The factor 'sigma' will only attach itself to the complex DNA / RNA polymerase when the RNA polymerase is attached to a promoter. Another hypothesis is that the factor sigma attaches to RNApol anyway and the enzyme is then able to slide along the DNA until it finds a promoter. It prevents detaching and speeds up promoter location, and decreases the affinity of RNApol for general regions of DNA. " Therefore, the answer seems to be, RNA attaches to DNA through a small protein called the factor sigma once the RNA polymerase attaches itself to a chain sequence called a "promoter". according to information from http://www.rothamsted.ac.uk/notebook/courses/guide/trans.htm " if the RNA polymerase attaches to a special sequence called a promoter, an additional small protein, the factor sigma, will also attach to the polymerase and lock it on the DNA. The factor 'sigma' will only attach itself to the complex DNA / RNA polymerase when the RNA polymerase is attached to a promoter. Another hypothesis is that the factor sigma attaches to RNApol anyway and the enzyme is then able to slide along the DNA until it finds a promoter. It prevents detaching and speeds up promoter location, and decreases the affinity of RNApol for general regions of DNA. " Therefore, the answer seems to be, RNA attaches to DNA through a small protein called the factor sigma once the RNA polymerase attaches itself to a chain sequence called a "promoter". role of sigma

Actually RNA Polymerase can bind to DNA anywhere in the entire genome but sigma factor attaches to polymerase only when it is at promotor. sigma factor dissociates when polymerase crosses promotor. sigma factor stablises the pre initiatiation complex. Actually there are many promoter and many genes but which gene to be transcribed is decided by sigma factor.