The first step toward gene expression is the transcription of a DNA template into a complementary RNA strand. This process is done by RNA polymerase, which reads the DNA template and produces an antiparallel RNA copy. As in DNA replication, the complementary strand is produced 5'->3'If the DNA template encodes for a gene, this RNA transcript will be refined into mRNA, which is further translated into a functional protein, as well, the transcribed template may also go on to make ribosomal RNA (rRNA) or transfer RNA (tRNA). The entire process can be broken into three major steps: initiation, elongation, and termination.
Initiation of transcription occurs differently in eukaryotes and prokaryotes. In eukaryotes, the transcription initiation complex must be formed. This includes, the core promoter, transcription factors, RNA polymerase, and activators/repressors. In prokaryotes, RNA polymerase and sigma factors are needed.
Promoters that do not rely on input and depend only the level of free RNA polymerase holoenzyme are referred to as constitutive. These are always turned on and
These promoters depend on the level of transcription factors that are not sigma factors. As the concentration of activator increase, the rate of transcription also increases.
Negatively regulated promoters on the level of a repressor transcription factor. Increased levels of a repressor will lower the activity of these promoters.
Promoters in this category are either positively or negatively regulated by multiple transcription factors. These are most useful when a promoter that relies on multiple environmental factors to function is desired.