Transcription factors and initiation of RNA synthesis in eukaryotes
For eukaryotic RNA synthesis, the regulatory DNA sequences (promoters, enhancers, silencers, etc.) for genes transcribed by each of the three RNA polymerases differ. A number of transcription factors are also involved in the formation of a transcription complex, needed for initiation of transcription. Each RNA polymerase is believed generally to have its own set of transcription factors, although TFIID or a part of it (TBP = TATA binding protein)is required for all the three RNA polymerases (see later). These transcription factors may be defined as "proteins, which are needed for initiation of transcription, but are not a part of RNA polymerase". The regulatory DNA sequences and the transcription factors used by RNA polymerase II (used for mRNA synthesis) may be either common for many genes expressed constitutively or may be specific for tissue or stimulus specific expression of genes, whose expression is regulated. It means that for transcription of different genes, no single combination of DNA sequences or factors is adequate and essential, and that the transcription may be initiated in many different ways.