Transcription factors are protein complexes that help RNA polymerase bind to DNA. RNA polymerase is the enzyme that transcribes genes to make messenger RNA, which is then used to make protein. By controlling RNA polymerase's access to the gene, transcription factors control the rate at which a gene is transcribed. Without transcription factors, cells would not be able to effectively regulate the rate at which genes are expressed.

Basal Transcription Factors Bind to the Gene Promoter Region

Every gene has a region known as the promoter. This is a DNA sequence "upstream" from the coding region, to which RNA polymerase must bind before it begins transcribing the coding region of the gene. In eukaryotes, the promoters of many (but not all) genes contain the sequence TATAA twenty-five to thirty nucleotides upstream from the transcription start site (T is the nucleotide adenine; A is thymine). Called the...