General transcription factors (GTFs), also known as basal transcriptional factors, are a class of proteintranscription factors that bind to specific sites on DNA to activate transcription. GTFs, RNA polymerase, and the mediator multiple protein complex constitute the basic transcriptional apparatus. GTFs are also intimately involved in the process of gene regulation, and most are required for life.
In archaea and eukaryotes, transcription initiation requires an RNA polymerase and a set of multiple GTFs. The Transcription initiation by eukaryotic RNA polymerase II involves the following GTFs:
A sigma factor is a protein needed only for initiation of RNA synthesis in bacteria. Sigma factors provide promoter recognition specificity to the RNA polymerase (RNAP) and contribute to DNA strand separation; they then dissociate from RNA polymerase core enzyme following transcription initiation. The mechanism is RNA polymerase core associates with the sigma factor to form RNA polymerase holoenzyme. Sigma factor reduces the affinity of RNA polymerase for nonspecific DNA while increasing specificity for promoters, allowing transcription to initiate at correct sites. The core enzyme of RNA polymerase has five subunits (protein subunits) (~400 kDa). Because of the RNA polymerase association with sigma factor, the complete RNA polymerase therefore has 6 subunits: the sigma subunit-in addition to the two alpha (α), one beta (β), one beta prime (β'), and one omega (ω) subunits that make up the core enzyme(~450 kDa). In addition, many bacteria can have multiple alternative σ factors. The level and activity of the alternative σ factors are highly regulated and can vary depending on environmental or developmental signals.