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shikimate accumulation. inhibition of synthesis of aromatic amino acids and
secondary metabolites causing cell death.
2.6.2.3. Cyanamide hydratase
(cult)
gene
The cah gene coding for the enzyme cyanamide hydratase (urea hydrolase;
E.C.
4.2.1.69) has been isolated from the soil fungus Myrorhecium verrucaria (Maier-
Greiner et al., 1991a). Cyanamide hydratase catalyzes the hydration of the nitrile
group of cyanamide to form urea, which can be used for plant growth. Cyanamide is a
nitrile derivative that in its aqueous or calcium salt forms can be used as a fertilizer. It
has the additional characteristics of acting as a non-persistant herbicide when applied
prior to seed germination. The enzyme has extremely narrow substrate specificity.
The use of cyanamide hydratase as a selectable marker has been demonstrated in
wheat (Weeks et al., 2000). tobacco (Maier-Greiner
et
al., 1991b), potato, tomato, rice
and Arabidopsis (Damm, 1998)
2.6.3. Marker genes for positive selection
Some marker genes for positive selection make possible the identification and
selection of genetically modified cells without injuries or death of the non-
transformed population of cells (negative selection). In this case, the selection marker
genes should confer, to the transformed cells, the capacity to metabolize some
compounds that are not usually metabolized. This fact will give to the transformed
cells an advantage over the non-transformed ones. The addition of new compound in
the culture medium, as nutrient source during regeneration process. allows normal
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