Maize alcohol dehydrogenase promoter (Adh)
Some plants, e.g., rice, have the ability to synthesize proteins that allow them to thrive under transient anaerobic conditions. One of those proteins is alcohol dehydrogenase, which is involved in ethanolic fermentation.
In maize, there are two proteins identified: alcohol dehydrogenase I (ADH-I) and II (ADH-II), of which ADH-I is the most important because its gene promoter has been widely used. In the 5′ untranslated regions of the maize ADH genes, anaerobic regulatory elements and an intron are important for driving gene expression in monocots. The regulatory elements responsible for the anaerobic response of the genes are within a 247 bp segment immediately upstream of the CAP site, more specifically between positions -140 and -99 of the maize Adh-1 promoter. Within this 40 bp segment, there are two essential regions, each of around 15 bp, required for expression under low oxygen conditions.
The whole Adh-1 promoter has been used in cereals such as rice, oat and barley and in dicots such as tobacco to drive the expression of genes of interest, providing very low levels of expression in the dicot crop. The Adh-I promoter in conjunction with the intron has proved to be much superior for transformation of some cereals such as rice.
Only two regions of the regulatory transcription region of the maize ADH I gene is currently used to drive gene expression in transformed plants, mainly monocots:
- the anaerobic regulatory elements (AREs) of the Adh-1 promoter, and
- the first intron of the maize Adh-1 gene.
The two elements are the basis of a recombinant promoter construct called pEmu. pEmu is formed in the 5′ to 3′ direction by:
- a truncated Adh-1 promoter having multiple copies of ARE,
- enhancer elements from the octopine synthase (OCS) gene of A. tumefaciens,
- a TATA box from a promoter expressed in plants, and
- the maize Adh-1 intron 1.
The promoter construct has been tested in transient assays in various monocot crops, driving a very high level of gene expression compared to the 35S CaMV promoter. It has also been used for the development of transgenic rice and sugarcane. However, in some stable transformations, the response has been poor and the recovery of the transgenic plants difficult. Problems have been attributed to the presence of duplicated OCS enhancer elements.