Genetic Manipulation - Environmental Biotechnology: Theory and Application - page 219

Figure9.3 RecombinantBaculovirus
Expression vectors
These are similar to the vectors described above but in addition have the required
signals located before and after the 'foreign' gene which direct the host cell to
translate the product of transcription into a protein. It is sometimes a difficult,
expensive or time consuming procedure to analyse for product from the 'foreign'
gene and so, in addition to the selector genes described above, there are frequently
reporter genes to indicate whether or not the signals are 'switched on' allowing
the 'foreign' DNA to be expressed. There are many reasons which are difficult
to predict, why even a perfectly constructed gene may not be functional, such as
the consequence of the exact site of insertion in the genome; hence the need for
inbuilt controls.
Reporter genes
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)