Synthetic Biology Nat. Rev. Genetics

Question 1

Bacteriophage λ lysis-lysogeny switch

Answer 1

Naturally occurring NOR gate.

Consists of two promoters that are each represse by the gene product of the other: The cro locus produces a product that suppresses the cl promoter, and the cl promoter produces a gene product that suppresses the cro promoter.

Once a transition from cro to cl is made, the promoter of cro is suppressed, and thus the gene has a state memory.

Question 2

Genetic Toggle

Answer 2

The P_L promoter from bacteriophage λ drives the transcription of lacI. The gene product of lacI represses a second promoter, Ptrc2 (a lac promoter variant). Ptrc2, in turn, drives expression of cl-ts, a gene encoding the temperature sensitive λ Cl repressor protein, which inhibits the P_L promoter.

This produces a toggleable expression system, which may be toggled in the P_L direction by the addition of isopropyl-beta-D-thiogalactopyranoside (IPTG) and in the Ptrc2 direction by a transient increase in temperature. A gene-of-interest or reporter gene may be placed under the P_L promoter and, upon removal of these exogenous signals, the system retains a memory of its current state.

Question 3

Circadian Clock

Answer 3

A naturally occurring, time-dependent, genetic oscillator.

In humans, the PER genes (period) and CRY genes (cryptochrome) produce products which enable CLOCK and BMAL1 to enter the nucleus and suppress the PER and CRY promoters. This process occurs in a time-dependent manner and may receive input from various other sources to calibrate the clock as the cycle progresses.