transcription factor, molecule that controls the activity of a gene by determining whether the gene’s DNA (deoxyribonucleic acid) is transcribed into RNA (ribonucleic acid). The enzyme RNA polymerase catalyzes the chemical reactions that synthesize RNA, using the gene’s DNA as a template. Transcription factors control when, where, and how efficiently RNA polymerases function.
Transcription factors are vital for the normal development of an organism, as well as for routine cellular functions and response to disease. Transcription factors are a very diverse family of proteins and generally function in multi-subunit protein complexes. They may bind directly to special “promoter” regions of DNA, which lie upstream of the coding region in a gene, or directly to the RNA polymerase molecule. Transcription factors can activate or repress the transcription of a gene, which is generally a key determinant in whether the gene functions at a given time.
Basal, or general, transcription factors are necessary for RNA polymerase to function at a site of transcription in eukaryotes. They are considered the most basic set of proteins needed to activate gene transcription, and they include a number of proteins, such as TFIIA (transcription factor II A) and TFIIB (transcription factor II B), among others. Substantial progress has been made in defining the roles played by each of the proteins that compose the basal transcription factor complex.
During development of multicellular organisms, transcription factors are responsible for dictating the fate of individual cells. For example, homeotic genes control the pattern of body formation, and these genes encode transcription factors that direct cells to form various parts of the body. A homeotic protein can activate one gene but repress another, producing effects that are complementary and necessary for the ordered development of an organism. If a mutation occurs in any of the homeotic transcription factors, an organism will not develop correctly. For example, in fruit flies (Drosophila), mutation of a particular homeotic gene results in altered transcription, leading to the growth of legs on the head instead of antenna; this is known as the antennapedia mutation.
Transcription factors are a common way in which cells respond to extracellular information, such as environmental stimuli and signals from other cells. Transcription factors can have important roles in cancer, if they influence the activity of genes involved in the cell cycle (or cell division cycle). In addition, transcription factors can be the products of oncogenes (genes that are capable of causing cancer) or tumour suppressor genes (genes that keep cancer in check).
Transcription factors function in the nucleus, where genes are found, and nuclear transport (i.e., import or export) of transcription factors can influence their activity. Another important general mechanism controlling the activity of transcription factors is posttranslational modification such as phosphorylation. Finally, in addition to controlling the genes and transcription of other transcription factors, these protein complexes can also control the genes responsible for their own transcription, leading to complex feedback control mechanisms.