Transcription in Prokaryotes

• RNA Structure


  Contains Ribose and Uracil

  Degrades rapidly and is usually single stranded

  Often forms of secondary structure
• Classes of RNA

  mRNA, rRNA,tRNA,crRNA,tracrRNA
• RNA is transcribed from one DNA Strand
• Transcription

  Synthesis of RNA from a DNA template

  This requires: DNA template, Raw Materials (rNTPS), and proteins that catalyze RNA synthesis
• Transciption Unit


  The stretch of DNA that is transcribed into an RNA molecule
• Promoter


  Specific region of a gene where RNA polymerase can bind and begin transcription. This determines the start site.
• Terminator


  Sequence of nucleotides that signals the end of transcription

  Is transcribed

  May require a protein for termination
• Upstream


  Toward the 5' end
• Downstream


  Toward the 3' end
• +1

  The first nucleotide transcribed
• RNA Polymerase


  Bacterial cells have only one type

  Large ,multi-subunit enyzme

  • 2 copies of a
  • 1 copy each of B and B'
  • 1 optional copy of w (stabilizes the enzyme)
• Sigma Factor


  Controls the binding of RNA polymerase to the promoter
• Process of transcription

  Initiation, Elongation, and Termination
• Initiation of Transcription

  Promoter recognized

  Bubble Formed

  Binding of rNTPs

  Movement of transcription

  Apparatus off promoter
• Bacterial Promoters

  Promoters have short stretches of DNA that are very similar

  • Consensus sequences

  -10 consensus sequence

  • Pribnow Box
  • 5'-TATAAT-3'

  -35 consensus sequence

  • 5'-TTGACA-3'
• Actual -10 and -35 sequences vary from the consensus sequence
• Upstream Element

  Sigma factor binds to RNA Polymerase

  • Holoenzyme
  • Binds to the -35 and -10 consensus sequence of promoter
  • Alpha subunit interacts with upstream element

  RNA Polymerase unwinds DNA from -12 to +2
• Imitation of Transcription

  Consensus sequences position RNA Polymerase at position +1

  RNAP pairs a free rNTP with its complementary base on the DNA (Keeps all three phosphates on 5' end)

  Additional rNTPs are added
• Initiation to elongation

  When RNAP synthesizes 9-12 nucleotides it undergoes a conformational change

  Enzyme released from promoter

  Sigma factor released from enzyme