Eukaryotic gene expression requires RNA polymerase in addition to five proteins called general transcription factors. These proteins interact with RNA polymerase II, allowing it to bind to DNA promoter sequences and initiate messenger RNA synthesis. General transcription factors are necessary for gene transcription and all promoters activated by RNA polymerase II. The best characterized general transcription factor is TFIID, which binds to a DNA promoter sequence called the TATA box.
In contrast to the vast majority of genes, the genes encoding ribosomal and transfer RNA are transcribed by RNA polymerases I and III respectively. These polymerases and their corresponding gene promoters require a different set of general transcription factors than RNA polymerase II.
Various other transcription factors help regulate the expression of specific genes. They often consist of a steroid hormone bound to its protein receptor. The hormone-receptor complex enters the nucleus where it binds to DNA promoters and/or enhancer sequences, thereby increasing or decreasing gene transcription. Steroid hormones include cortisol, aldosterone, estrogens, progesterone, testosterone and vitamin D.
Peptide hormones do not cross cell membranes, but they can still affect gene transcription. This chain of events starts when the hormone binds to a cell surface receptor leading to tyrosine kinase activation; an influx of calcium; or the formation of a second messenger, such as cyclic adenosine monophosphate (cAMP). Tyrosine kinase's phosphorylate transcription factors associated with mitosis include MAP kinase. Calcium and cAMP activate signal transduction pathways that result in gene transcription via Protein Kinase C and CREB (Cyclic AMP Response Element Binding Protein).