Hygromycin phosphotransferase (hpt) gene
The hygromycin phosphotransferase (denoted hpt, hph or aphIV) gene was originally derived from Escherichia coli. The gene codes for hygromycin phosphotransferase (HPT), which detoxifies the aminocyclitol antibiotic hygromycin B. A large number of plants have been transformed with the hpt gene and hygromycin B has proved very effective in the selection of a wide range of plants, including monocotyledonous. Most plants exhibit higher sensitivity to hygromycin B than to kanamycin, for instance cereals. Likewise, the hpt gene is used widely in selection of transformed mammalian cells.
The sequence of the hpt gene has been modified for its use in plant transformation. Deletions and substitutions of amino acid residues close to the carboxy (C)-terminus of the enzyme have increased the level of resistance in certain plants, such as tobacco. At the same time, the hydrophilic C-terminus of the enzyme has been maintained and may be essential for the strong activity of HPT.
HPT activity can be checked using an enzymatic assay. A non-destructive callus induction test can be used to verify hygromycin resistance. The antibiotic hygromycin B should be handled with care because it is toxic to humans.
Agricultural industrial applications
Field trials and commercial releases
Like the nptII marker gene, hpt has been used as a selectable marker gene for transgenic plants, but it is not an agronomic trait of interest in modified plants. It is one of the introduced genes needed to accomplish the production of transformed plants.
According to information provided by BioTrack, a database administered by the Organisation for Economic Cooperation and Development (OECD) containing records of field trials and commercial releases in OECD Member countries (currently 30), several cultivars of rapeseed, alfalfa and canola submitted to field trials in the United States and Canada between 1989 and 1996 contained hygromycin phosphotransferase as a selectable marker gene.
In Australia, several varieties of barley, wheat, grapevine, Indian mustard and poppy, engineered for viral tolerance, fungal resistance, improvement of fruit quality and insect resistance contain a hygromycin phosphotransferase gene. These crops are being tested in field trials and have not been commercially released yet.