The efficiency of transgene expression in plants is dependent on a number of factors that
affect mRNA accumulation and stability. In addition to the promoter (discussed in detail in
Chapter 9), these include untranslated sequences (UTRs) both upstream (5^0 ) and down-
stream (3^0 ) of the gene, codon usage, cryptic splice sites, premature polyadenylation
sites, and intron position and sequence (these important features affecting gene expression
are discussed in more detail in Chapter 6). Careful consideration of these important factors
should be made when designing vectors for transgene expression in plants. Once a decision
has been made as to whether a transgene should be expressed ubiquitously or cell-type-
specifically, inducibly or constitutively, by changing the promoter fragment used, further
decisions can be made that determine whether a gene product is required at high or low
levels. Often the omega sequence from the 5^0 UTR of the tobacco mosaic virus (TMV)
is used to enhance translation in plants. Omega contains a poly(CAA) region, which serves
Figure 7.9.Cartoon of a generic plant promoter. Typically, transcription factors bind promoter
sequences initiating the formation of the transcription complex. Components of the transcription
complex bind the CAAT box and the TATA box and assist with the recruitment of RNA
polymerase II, allowing the initiation of transcription. The transcription complex can cause the
DNA to bend back on itself, bringing together regulatory sequences far from the site of transcription.
7.2. DNA VECTORS 169