HIV-infected T cell FLICKR, NIAID HIV leads to AIDS primarily because the virus destroys essential immune cells called CD4 T cells, but precisely how these cells are killed has not been clear. Two papers published simultaneously today (December 19) in Nature and Science reveal the molecular mechanisms that cause the death of most CD4 T cells in lymphoid tissues, the main reservoir for such cells, during infection.
Transcription is carried out by an enzyme called RNA polymerase and a number of accessory proteins called transcription factors. Transcription factors can bind to specific DNA sequences called enhancer and promoter sequences in order to recruit RNA polymerase to an appropriate transcription site. Together, the transcription factors and RNA polymerase form a complex called the transcription initiation complex.
Each RNA Pol is responsible for a different class of transcription : PolI transcribes rRNA (ribosomal RNA), PolII mRNA (messenger RNA), and PolII tRNA (transfer RNA) and other small RNAs. Any protein that is needed for the initiation of transcription is defined as a transcription factor. Many transciption factors act by recognizing cis-acting sites that are parts of promoters or enhancers. However, binding to DNA is not the only means of action for a transcription factor. A factor may recognize another factor, or may recognize RNA Polymerases. In Eukaryotes, transcription factors, rather than the enzymes themselves, are principally responsible for recognizing the promoter.