Team:Buenos Aires/Project/Schemes

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Schemes

Scheme 1: Crossfeeding

In the cross-feeding scheme we plan to use strains which are auxotrophic for certain amino acids (e.g. lysine and tryptophan). By starting with a strain with both auxotrophies, A will be created by rescuing the biosynthetic pathway for Trp and forcing its export from the cell. Analogously, we will create strain B by rescuing the Lys pathway and forcing the secretion of Lys from the cell.

Amino-acid export from the cell can be engineered in the form of peptides (enriched for the relevant amino acid) which, in turn, can be easily controlled at the transcriptional and translational level. Export will be done either with secretion tags or as «trojan peptides» (Derossi et. al., 1998), which can diffuse through the plasma membrane. The main tuneability of the system could be changing the export rates for each amino acid. Because of the availability of auxotrophic mutants in yeast, this chassis is ideally suited for the crossfeeding circuit design.

Scheme 2: Independent Population Control

In this scheme, quorum sensing (QS) controls the density of each strain in an independent manner.

The system has already been implemented for a single strain, using the LuxR/LuxI system from Vibrio fischeri, and the CcdB killer protein (E in the figure) under control of a LuxR responsive promoter (You et. Al. 2004). This strain was shown to stabilize its growth at a cell density below saturation.

Our idea is to extend the system, creating new strains with the same overall functioning but different autoinducers so as to avoid interference with each other. Tuneability can be achieved by modulating both production and sensitivity to the autoinducer.

Scheme 3: Cross-population Control

This scheme is a modification of the «independent population control», where the autoinducers produce some effect on the other strain(s) beside modulating the growth of the strain that produces it. This coupling between strains can increase robustness and give rise to some interesting behaviors as oscillations.

As more strains are added, several different connections between them can be implemented that will probably result in different properties of the system.

Scheme 4: Stochastic State Transitions

This scheme is quite different to the previous ones in the sense that there is a single strain that has several stable states (e.g. A and B). Each state activates the transcription of different sets of genes. The transition between states is given in a stochastic manner, but the probability of transition can be externally modulated. Each cell will spend a fraction of the time in each state independently of other cell.

Therefore, at any given time, cell population will be composed of a fraction of cells in each state. Tuning the transition rates between states will result in a correspondent change in the fraction of cells in each state. The system can be implemented as a «leaky toggle switch»

Pros and cons:

+ Completely independent of cell density.

+ If a rare state induces death, a slow population growth could be achieved.

- All components need to be incorporated in the same strain, thus no circuit integration or isolation is possible (see applications section).