- Always on by default unless you add the thing that would normally be synthesized in the medium (ex:
A.A.)
Lactate - ✔️✔️organic acid form of lactose
Catabolic pathways - ✔️✔️* Degradation of lactate &/or arabinose
- Typically off by default & only turned on when the thing they can grow on is in the cell
Housekeeping genes - ✔️✔️are constitutive. They perform core cellular functions and so are always
expressed in cells no matter the conditions
Inducible genes - ✔️✔️are genes that code for enzymes that are induced, either by the binding of an
inducer around a promoter or by the dissociation of an inducer from the promoter region.
- Off by default
- Catabolic pathways are usually inducible - you don't express enzymes to use specific sugars as electron
donors unless those sugars are present!!
Includes electron donors & acceptors
Repressible genes - ✔️✔️* are genes whose expression is shut down (repressed) by the production of a
product in the metabolic pathway they are expressed in.
- Anabolic (biosynthetic) pathways are usually repressible. Why express a tryptophan biosynthesis
pathway is tryptophan is already available?
Transcriptional regulatory proteins - ✔️✔️* are proteins that bind directly to DNA sequences to stimulate
or shut off (they interact with nitrogenous bases through the major groove of DNA)
- These proteins usually have a helix-turn-helix motif (α helix - β sheet - α helix)
- Sequence specific
, negative transcriptional control - ✔️✔️* binding of a protein to an operator (a region that is just
downstream of promoters) inhibits transcription
- These proteins are called repressor proteins; make sure RNA poly can't bind & transcribe genes, keep
pathways off
Positive transcriptional control - ✔️✔️* binding of a protein to activator-binding sites upstream of the
promoter stimulates transcription
- These proteins are called activator proteins; help keep pathways on
What are most metabolic pathways? - ✔️✔️have negative and positive control
Negative control (inducible genes) - ✔️✔️* Binding of an inducer causes the repressor protein to
dissociate - transcription can begin!
* The inducer is usually the substrate for the catabolic pathway, e.g., D-mannose as a carbon source and
electron donor
Positive control (inducible genes) - ✔️✔️* Notice that the activator-binding site is upstream of the
promoter
* Binding of an inducer to the activator protein causes the activator protein to bind to the activator-
binding site, which allows transcription to occur!
Negative control (repressible genes) - ✔️✔️* Default stage is on
* Repressor protein (aporepressor) isn't bound to the operator, so transcription can proceed
* Binding of the corepressor to the aporepressor protein causes the repressor protein to bind to the
operator and shut down transcription
* The corepressor is usually the final product in a biosynthetic pathway, e.g., an amino acid like
tryptophan
Positive control (repressible genes) - ✔️✔️* Off by default
* Transcription proceeds while the activator is bound to the activator binding site
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