Biochemistry: Transcription in Prokaryotes

Study Notes

Two genes being transcribed (rRNA genes) can be visualised by electron microscopy
Multiple transcripts from each gene are transcribed concurrently
Direction of transcription can be determined

Three stages of transcription: Initiation, Elongation, and Termination
E. coli Promoters:
- Required to recruit and position RNA polymerase
- Usually don't know where to start
- Sigma factor required for sequence-specific DNA binding
- Sigma factor binds RNA polymerase and dissociates after initiation
Bacterial Transcription Cycle:
- RNA polymerase unwinds DNA and starts copying the template strand
- Sigma factor releases

Gene expression is regulated mostly at the level of transcription
Most transcriptional regulation occurs at the level of initiation
Many copies of mRNA are transcribed with short half-lives
Advantage of having many copies of short-lived RNA molecules: allows for rapid response to changing environment

Classic example of transcriptional control in E. coli
Purpose: to hydrolyse lactose so glucose can be used for energy
Multiple genes regulated together:
- β-galactosidase
- Permease
Lac Operon - Overview:
- Lac repressor is active in its native state (binds DNA and represses transcription)
- Allosteric binding of allolactose prevents DNA binding
- CAP protein is inactive in its native state (cannot bind DNA) and requires allosteric binding of cAMP for DNA binding
Lac Operon by condition:
- No lactose, no glucose: operon repressed
- With lactose, no glucose: operon activated
- No lactose, with glucose: operon repressed
- With lactose, with glucose: operon weakly active

Binding of CAP and the lac repressor is sequence-specific, non-covalent, and reversible
Even when the operon is repressed, a little bit of transcription still occurs
Activators recruiting RNA polymerase/sigma factor is a common control mechanism in bacteria (e.g. CAP in the lac operon)
Cooperative binding is a key concept in biochemistry (e.g. binding affinity of 2 > sum of each individual)