Prokaryotic Transcription

• Transcription transfers information from DNA to RNA
• Similarities to replication:

  • 5' to 3' synthesis
  • Method of elongation
  • Reaction is driven by the breakdown of pyrophosphate
• Differences to replication:

  • No RNA primer is required
  • No proof-reading function
• Messenger RNA = mRNA
• mRNA encodes proteins
• Transfer RNA = tRNA
• tRNA carries amino acids
• Ribosomal RNA = rRNA
• rRNA forms the ribosome, which catalyses protein synthesis
• In prokaryotes, mRNA, tRNA and rRNA are made by the same RNA polymerase
• Only one strand of DNA is transcribed at a single time
• The template (antisense) strand is complementary to the RNA transcript
• The coding (sense) strand has the same sequence as the RNA transcript, but with T instead of U
• Requirements for Transcription

  1. DNA template
  2. Ribonucleoside triphosphates
  3. RNA polymerase
• The promotor is a DNA sequence that directs RNA polymerase to the correct site to begin transcription
• Transcription is initiated at the promotor site
• TTGACA and TATAAT are consensus sequences for prokaryotes
• In bacteria, there are two common promotor sequences at the -10 and -35 positions
• The efficiency or strength of a promotor sequence dictates the frequency of transcription
• The distance between promotor sequences also alters their effectiveness
• Strong promoters have sequences that closely resemble the consensus sequence
• Promotors are recognised by the RNA polymerase sigma subunit
• The sigma subunit decreases the affinity of RNA polymerase to DNA, allowing RNA polymerase to slide along DNA looking for the promotor sequence
• Once transcription is initiated, the sigma subunit is released from RNA polymerase and free to participate in another round of initiation
• RNA polymerase unwinds DNA to form a transcription bubble
• RNA synthesis begins without the need for a primer
• The RNA strand is synthesised in the 5' to 3' direction
• The first base of RNA synthesis is either A or G, as they base-stack more effectively
• Elongation proceeds within a transcription bubble
• RNA polymerase has proofreading activity when in the presence of accessory proteins
• RNA synthesis is more error prone than DNA synthesis
• The new RNA strand forms a temporary hybrid helix with the DNA strand template within the transcription bubble
• Transcription stops due to signals within the newly synthesised RNA
• Termination signals can be intrinsic or protein dependent
• Intrinsic termination signal

  • The RNA anneals to itself, forming a hairpin
  • RNA polymerase is stalled, causing the DNA to bind its normal coding strand and close the transcription bubble
  • RNA polymerase releases DNA
• a hairpin forms due to a GC-rich region followed by several U
• Protein dependent termination signal

  • Rho proteins bind to a rich C but poor G sequence
  • Rho proteins slide along the chain until they hit RNA polymerase, dislodging RNA from the DNA template
• In bacteria, the expression of functionally related genes are controlled together as an operon
• Bacteria use glucose as their main source of energy, but will use lactose if necessary
• In order to use lactose, the bacteria must produce a set of enzymes to break down and transport lactose