general transcription factors and the RNA polymerase assemble. The most important feature of promoters of genes transcribed by RNA polymerase II is the TATA box, which serves as the starting point for the assembly of the general transcription factors. The point at which RNA polymerase begins transcription is typically located about 25 nucleotide pairs downstream from the TATA box. The gene regulatory sequences serve as binding sites for gene regulatory proteins whose presence on the DNA affects the rate of transcription initiation. The regulatory sequences can be located adjacent to the promoter, far upstream of it, or even downstream of the gene. DNA looping allows the gene regulatory
proteins bound at sites distant from the promoter to interact with the proteins that assemble at the promoter.
Whereas the general transcription factors that assemble at the promoter are the same for all genes transcribed by polymerase
II, the gene regulatory proteins and the locations of their binding sites relative to the promoters are different for different genes. The effects of multiple gene regulatory proteins combine to determine the rate of transcription initiation. However, it is not yet understood in detail for any gene how these effects are combined.