There are many such genes in common use and these usually code for an enzyme. The most common is β-galactosidase, mentioned above. This enzyme, supplied with the appropriate reagents, may also catalyse a colour change by its activity on a variety of chemical compounds typified by orthonitrophenolgalactoside (ONPG) which changes from colourless to yellow on hydrolysis in much the same way as the blue/white screening described above for the cloning vector, pGEM . Other reporter genes produce enzymes which can cause the emission of light such as the luciferase isolated from fireflies, or whose activity is easy and quick to assay like the bacterial β-glucuronidase (GUS), which is probably the most frequently used reporter gene in transgenic plants. Reporter genes can only be a guide to the process of transcription and translation occurring in the cell and it has been acknowledged for some time that care must be exercised to avoid misinterpreting data (Pessi, Blumer and Haas. 2002).
As with selector genes, the reporter genes serve no useful purpose once the cloning procedure has been successfully accomplished to produce the finished product. In the early days of this technology, these genes would normally be left in situ to avoid the extra work of removing them which might also upset the structure of the recombinant genome thus diminishing the quality of the carefully engineered organism. There is, however, an argument to remove all genes which were necessary for construction purposes but which no longer serve a useful purpose, to reduce perceived potential risk of unwittingly increasing the spread of genes throughout the environment.
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