Using a reverse genetics strategy, the gene for phytoene synthase
(Psy) was identified in transgenic tomato plants in which the
expression of a candidate cDNA was silenced [7]. A unique
functional complementation approach to cloning genes has
been developed on the basis of the ability of the
carotenogenic enzymes to function in the bacterium
Escherichia coli [8]. This so-called ‘color complementation’
technique takes advantage of E. coli engineered with bacterial
genes to produce a colored carotenoid that serves as a precursor
for the enzyme under investigation. The carotenoid accumulated
in the bacteria imparts a characteristic color to the
colonies that can be seen by the naked eye. The screening for
a specific gene is based on the visualization of color changes
in E. coli colonies following transfection of the bacteria with
plant cDNA libraries carried on expression plasmids.