Promoters OR Enhancers -

It is needless to emphasize that for an efficient expression in plant cells, foreign genes (more particularly, prokaryotic genes) must have an appropriates promoter, 5'-leader and 3/-terminator sequences. A suitable enhancer sequence will also be needed if the gene is required to be expressed either in a specific tissue, during a specific developmental stage or in response to a specific stimulus.

A variety of promoter sequences have been used to drive genes in plant cells. These include the nos (nopaline synthase), ocs (octopine synthase) and mas (mannopine synthase) promoters from Agrobacterium, and 35S RNA gene promoter of cauliflower mosaic virus (CaMV).

Expression of many genes is confined to specific tissues and/or is induced by specific stimuli; such genes are called tissue-specific or stimulus­ responsive genes, respectively. Such specifities in gene expression are due to certain DNA sequences, called enhancers/silencers, either lying within or at a considerable distance (up to several kilobases) away from the promoters they affect.

An enhancer may be defined as a DNA sequence, which increases the activity of promoter of a gene, while DNA sequences suppressing promoter activity are called silencers. Thus enhancer/silencer sequences regulate the activity of promoters but are not themselves involved in the promoter activity perse.

These sequences act on promoters located on their either side, vary considerably in size, and can exert positive (promotory) or negative (inhibitory) effects on promoter action. Clearly, the first intros of Adh1 and shrunken 1 genes of maize act as enhancers of 35S promoter of CaMV.