What is happening at the DNA level to put the observed response into action?
Ohme-Takagi and Shinshi, in 1995, were able to shed some light on this subject:
Studying ethylene inducible transcription of pathogenesis related genes in Nicotiana spp, Ohme-Takagi and Shinshi have identified an 11-bp conserved sequence in the 5' upstream region of these genes which they propose is the sequence essential for ethylene responsiveness. The sequence, which they call the GCC box, has the following sequence:
By making transgenic plants containing a chimeric promoter-beta-glucuronidase (Gus) reporter with two tandem copies of a GCC fragment (made up of a 47-bp fragment from the 5' upstream region of the beta-1,3-glucanase gene which contains two copies of the GCC box), they were able to show that it is in fact this sequence which confers ethylene responsiveness to whatever gene it is in the promoter of.
Furthermore, they were able to clone and characterize four genes from tobacco for proteins which specifically interact with the GCC box in the ethylene responsive elelment. These four proteins, of predicted molecular masses, 26.4, 25.5, 24.2 and 32.8 kD, are named Ethylene Responsive Element Binding Protein (EREBP) 1 through 4. The deduced amino acid sequence of each of these proteins shows a region of 59 amino acids which is rich in charged amino acids and is highly conserved among the four proteins. Through truncation experiments it was determined that this region was the one responsible for DNA binding activity. However, the sequence of this region shows no significant homology to binding domains of any known transcription factors, nor does it have a leucine zipper or zinc finger motif which are common in plant transcription factors. They were able to find regions of high homology to the conserved region in EREBPs in proteins of unknown function in lupine, Arabadopsis and rice, indicating that this region is highly conserved among plants. The region does have an acidic region and a putative nuclear target site which are characteristic of transcription factors.
This GCC box was thus shown to be a target for the ethylene signal transduction pathway. However, it cannot be the only one as other ethylene inducible genes (including those involved in ripening and senescence) have been shown not to contain the GCC box. Additionally, the mRNA encoding the EREBPs was found to increase in level in response to external ethylene but it is yet unknown exactly how these levels are regulated. (Ohme-Takagi and Shinshi).
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