Cytosine deaminase as a substrate-dependent negative selectable marker in Brassica napus

The enzyme cytosine deaminase, encoded by the codA gene, catalyzes the deamination of the nontoxic compound 5-fluorocytosine (5-FC) to the highly toxic compound 5-fluorouracil (5-FU). Cytosine deaminase activity is not found in higher plants and Brassica napus seedlings are unaffected by the presence of 5-FC in the growth medium. In codA-transformed B. napus seedlings, expression of cytosine deaminase results in a reduction of root and hypocotyl lengths, and a severe suppression of true leaf development. This phenotype is dependent on the presence of the 5-FC substrate and no effects an seen in plants grown in the absence of the substrate or in sibling plants lacking the transgene. The codA transformants have been assessed over three generations of growth and in each generation the transgene is stably inherited and confers the same 5-FC-sensitive phenotype. Transfer of 5-FC-sensitive seedlings to soil results in the restoration of normal growth in up to 100% of the seedlings. These results indicate that codA is a versatile dominant marker gene that can be used effectively in B. napus for substrate-dependent negative selection.