crop improvement. Frequently, viral promoters, such as the CaMV 35Spromoter, are used
to express genes constitutively and near-ubiquitously in transgenic plants. These can be
replaced by native plant promoters with similar expression profiles, such as actin or ubiqui-
tin promoters. The use of such promoters to express transgenes both ubiquitously and con-
stitutively may, however, cause unwanted secondary effects that might be avoided by
designing and constructing vectors to deliver tissue-specific or conditional gene expression.
For example, dwarfism is an agronomically important trait, which helps plants survive
Figure 7.21.Auxillary vectors provide restriction recognition sites for the conventional cloning of an
expression cassette with the desired promoter, ORF, and terminator sequences. The expression cas-
settes are flanked by 8-bp restriction recognition sites or homing endonuclease sites. Homing endo-
nuclease sites are extremely rare in natural sequences and can facilitate the assembly of several
expression cassettes within the single plant transformation vector pPZP RCS2.
7.4. VECTOR DESIGN 183