Transcription: Initiation

Transcription, like its sister processes in the central dogma of molecular biology, can be divided into three stages:

  1. Initiation:  The polymerizing unit (in this case, RNA polymerase) is directed to the site; that is, there is something on the site that both flags the polymerase to stop there, and orients it in the proper direction.
  2. Elongation:  This is where the polymerizing unit does it’s “job”; in this case, laying down nucleotides complementary to the template strand, and fusing the sugar-phosphate backbone to form a new RNA strand.
  3. Termination:  This is where the polymerase stops (hence, the name). Like in initiation, there needs to be some sort of flag to signal the stopping point.

This is our friend, prokaryotic RNA polymerase.  For initiation, the sigma factor, shown in blue, is what helps direct RNA polymerase to the proper point of transcription; it has high affinity for the -10 consensus sequence on most bacterial promoters.

Initiation of transcription is started by the binding of sigma factor to the -10 sequence of a promoter.  RNA polymerase, being positively charged, “roams” the bacterial genome, moving over the DNA looking for a place to start.  When it encounters sigma factor, the protein-protein interactions cause RNA polymerase to “stick” somewhat to that area.  If it is oriented properly, the tail end of RNA polymerase will dock down upon the -35 sequence of the promoter; this area is also “sticky” for RNA polymerase, which further restrains the polymerase to that site.  The more restrained the RNA polymerase is, the more likely it will stay there long enough to start unwinding DNA and “melting” the base-pairs apart; once this occurs, transcription can proceed..