In DNA, there are certain specific sequences that the RNA polymerase recognizes as the signal that they are to bind at that point and start transcription: promoters. A promoter usually has three parts: the DNA "start site," the "-10 sequence" that appears before the start site, and the "-35 sequence" that appears before the -10 sequence. The coding at the -35 sequence is TTGACA. One can consider these two pre-sites as a notice to watch for the start site, where the actual binding occurs.
