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of regulatory RNAs. Some of these regulatory elements are in proximity to the
coding region, while others may be located far away. The stability of messenger
RNA is influenced by signals within the mRNA that can influence the amount
of protein produced in the cytoplasm. It is crucial to obtain expression of the
inserted gene at appropriate times and levels, and in the targeted tissues.
A factor that may be important in maintaining the inserted DNA in the trans-
genic arthropod over time is the presence of “origins of replication” that regu-
late DNA replication of the chromosomes. If a transgene is inserted into a region
of the chromosome far from a site where an origin of replication occurs natu-
rally, the gene could be lost over time because it is not replicated. Alternatively,
the transgene may be lost due to DNA-repair mechanisms, especially when the
arthropod is placed into a stressful environment.
A protein-coding sequence from a prokaryote such as E. coli can be used to
form a DNA construct that will function in an arthropod if regulatory sequences
from an arthropod are used (regulatory sequences from prokaryotes do not
function in arthropods). Because regulatory sequences may vary from species to
species, the source of regulatory sequences chosen may be as important, or even
more important, as the source of the protein-coding sequences. Furthermore,
some regulatory sequences allow genes to be expressed only in particular tissues
or in response to particular stimuli (such as heat shock), whereas other genes are
expressed in most tissues most of the time. If it is important that the inserted
gene function in a tissue- or stimulus-specific manner, it is essential to identify
tissue- or stimulus-specific promoters.
Initially, the number of suitable regulatory sequences available for genetic
manipulation of arthropods was limited, but that is changing with the sequenc-
ing of diverse arthropod genomes. Project goals will dictate what type of regula-
tory sequences might be most useful. In some cases, a low constitutive production
of transgenic proteins will be useful, while in other cases high levels of protein
production will be required after inducement by a specific cue. Researchers may
have to evaluate the tradeoffs between high protein production and the subse-
quent effect these have on fitness of the transgenic arthropod strain based on
the specific goals of each program. The presence of introns sometimes may be
necessary for high levels of transgene expression in transgenic insects ( Duncker
et al. 1997 ).
14.9 How are Modified Arthropods Identified?
After inserting the desired genes and regulatory elements into the nuclear
genome of an arthropod, the next issue is how to detect whether the transgene
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