Regulatory Sequences Control Gene Expression
Enhancer and Silencer Elements
The Effect Of Ennhancer And Silencer Elements On Gene Expression
EnhancersCertain sequences have been described that effect the level of gene expression. We have already described some that have an up and down effect on expression. But other sequences have been found that have a dramatic effect on expression.
Enhancer- a DNA sequence that greatly increases the expression of a gene in its vicinity; the sequence can reside upstream or downstream and its orientation is not fixed; these sequences have been shown to have an effect as far away as 3000 bp
The effect of enhancers elements were studied in the following two papers: PNAS 83:8560 (1986) and EMBO Journal 7:297 (1988). Alpha-conglycinin is a seed storage protein of soybean that is constructed from several subunits. The genes for these subunits are expressed at high levels during specific stages of seed development. What effect do the different promoter sequences of these genes have on seed development. Again, as with other researchers, deletions of the promoter of the gene for one subunit (alpha') were constructed and analyzed in transgenic plants. The results were:
CaMV 35S Promoter --- CAT --- Alpha ' 3' end
The -78 to -257 region of the promoter was then inserted into different locations of this construct, and the effects on gene expression were monitored. The following results were obtained from transgenic tobacco plants.
SilencersThe experiments that analyzed the promoter of the alpha' subunit of the soybean beta conglycinin protein revealed that sequences were present that exhibited enhancer-like activities as well as temporal and spatial regulation. Spatial regulation is of particular importance, since, for example, there is no need to express a seed storage protein in the leaf of a plant or a protein involved in photosynthesis in the root of a plant. How is this negative regulation manifested? Again, reporter genes driven by different promoters were analyzed in transgenic plants.
The effects of a promoter element from the pea chlorophyll a/b binding protein gene on gene expression was reported in 1986 (Nature 323:551). Previous experiments had shown that kanamycin resistance could be obtained only if the first 400 bp of the promoter were present driving an NPT II resistance gene. If sequences from -100 to -347 were deleted then kanamycin resistance could not be obtained which suggested this region contained a positive regulatory element involved in tissue-specific or light dependent expression. This region was then introduced into transgenic plants linked to a reporter gene. The following constructs were made and their effect on gene expression are noted in the following table.
The following conclusions can be made from the results presented in the table.