Let me compare different transcription factor mechanisms:
Common Examples of Repressor TFs:
- LacI (lactose) – Classic repressor
- TetR (tetracycline) – Well-characterized repressor
- ArsR (arsenite) – Metalloregulatory repressor
- PurR (purines) – Metabolite-responsive repressor
- FadR (fatty acids) – Dual function but mainly repressor
Regarding RhaR/RhaS (rhamnose system): Actually, the rhamnose system works differently – it’s an activator system. The rhamnose transcriptional regulation involves:
- RhaR and RhaS are positive regulators
- In presence of rhamnose:
- RhaR activates rhaS transcription
- RhaS activates rhamnose catabolic genes
- Without rhamnose, very low transcription occurs due to lack of activation
This is fundamentally different from LldR because:
- LldR (repressor): high background requires better repression
- RhaS (activator): high background would require reducing promoter leakiness or improving activation specificity