Promoter sequences from 10 bacteriophage and bacterial genes

Figure 10-13 Catabolite control of the lac operon. (a) Onlyunder conditions of low glucose is adenylate cyclase activeand cAMP (cyclic adenosine monophosphate) formed.(b) When cAMP is present, it forms a complex with CAP(catabolite activator protein) that activates transcription bybinding to a region within the lac promoter.

NEGATIVE REGULATIONREPRESSIBLE TRANSCRIPTION

X

Aporepressor

Co-repressor

Active repressor

Operator

THE trp OPERON

1 2 3

4

21

3 4

RNA Polymerase Transcribing a Prokaryotic Gene

• Initiation occurs at a transcription start site in a promoter (DNA sequence)

• Termination occurs at a transcription stop site

• Activation of bacterial RNA polymerase requires binding of sigma factor

Transcription in Eukaryotes

Eukaryotic RNA Polymerases

• Three different RNA polymerases transcribe nuclear genes

• Other RNA polymerases found in mitochondria and chloroplasts

Eukaryotic vs. Prokaryotic Transcription

• In eukaryotes, transcription and translation occur in separate compartments.

• In bacteria, mRNA is polycistronic; in eukaryotes, mRNA is usually monocistronic.– Polycistronic: one mRNA codes for more than one polypeptide– moncistronic: one mRNA codes for only one polypeptide

• 3 RNA polymerases in euk., 1 in prok.

• Binding of Basal Transcription Factors required for euk. RNA Pol II binding.

• “Processing” of mRNA in eukaryotes, no processing in prokaryotes

Gene expression

TERMINATION• RNA polymerase meets the terminator

• Terminator sequence: AAUAAA

• RNA polymerase releases from DNA• Prokaryotes-releases at termination signal• Eukaryotes-releases 10-35 base pairs after

termination signal