Biology Reference
In-Depth Information
FIGURE 27.5
Use of synthetic biology techniques to test a hypothesis for forming distinct zones of gene
expression in response to a morphogen gradient. (a) Illustrates Lewis Wolpert's famous analogy for gradient-
determined gene expression, the French Flag, cells taking on the states represented by different colours of the flag in
response to the concentrations of signalling molecule, shown in the graph below. The dotted lines t1 and t2
represent thresholds. (b) A hypothetical feed-forward system, with 'arms' of different efficiency, that will switch on
the output only between t1 and t2 (see main text). (c) The system built by Basu and colleagues.
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(d) Sketch of the
response actually made by the engineered cells.
which represses it strongly, or LacI*, a weaker mutant. In principle, when AHL is present at
low concentrations, neither LacI* nor cI will be expressed; the absence of cI will allow LacI to
be transcribed efficiently, so GFP will be held 'off' by LacI's presence. When the concentration
of AHL is somewhat higher, LacI* and cI will be transcribed somewhat: enough for cI to hold
expression of LacI off, but not enough for the weaker repressor, LacI*, to suppress GFP effec-
tively. GFP will therefore be expressed. At higher concentrations still, there will be enough
LacI* to repress GFP despite the weak nature of the repressor, and GFP will therefore be
'off'. When real cells engineered to express the above system were cultured on a plate, and
a source of AHL placed in the middle, a discrete ring of GFP expression appeared at some
distance from the source, exactly as it should have done. When multiple AHL sources
were used, more complex patterns of GFP were observed, again as would be expected
according to the shapes of the gradients.
The example just described addresses patterning, not morphogenesis itself, but it does
serve to illustrate the power of the synthetic approach. How applicable might this approach
be to morphogenesis? The morphogenetic behaviours of mammalian cells can be divided,
for the purposes of discussion, into just a few basic activities. These include proliferation,
elective death, migration, directed migration, aggregation, mesenchyme-to-epithelial transi-
tion, sheet folding, sheet joining, convergent extension, tubulogenesis, branching and
