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Genetic Engineering of Entomopathogenic Microbes for Pest Management 257
production of viral DNA and a coat protein, followed by assembly of the virus particle,
and release of such particles from the cell. Transducing particles are formed by packaging
of the host DNA in place of the viral DNA. In conjugation, the transfer of DNA takes place
through cell-to-cell contact. It is controlled by plasmids, but can also be controlled by
transposones. Replacement with alternate plasmids results in a different spectrum of
genetic activity. Some plasmids can be transferred easily, while others require the presence
of transfer profi cient plasmids before genetic transfer can take place. These changes occur
in nature, where exchange of the genetic material is a common phenomenon (Walter and
Siedler, 1992).
Recent advances have made it easier to insert foreign DNA into organisms at will.
Through this process, bacteria capable of performing different functions have been produced,
for example, those producing mammalian proteins, hormones, or toxins from other
organisms. This may involve the transfer of genetic material to another microorganism or
it may be expressed differently, posing a new challenge. The exchange of DNA between
microbes may be infl uenced by conjugal plasmids, cloning vectors, transducing phases, or
transposones used, and the method used. Release of a number of genes into the environment
may affect the natural process of selection and evolution, and addition of a large
amount of genetic material can also increase the probability of mutations. To limit the
exchange of genetic material, plasmids have been produced by altering the nucleotide
sequences that are unable to function in the exchange process. However, under certain
circumstances, helper plasmids can enter the cell, resulting in the mobilization of the disarmed
plasmid (Zylstra, Cuskey, and Olson, 1992).
Genetic Engineering of Microbes
Several applications of genetic engineering of microbial pesticides will emerge in future.
Microorganisms producing insecticidal secondary metabolites can be genetically engineered
to produce such chemicals more effi ciently. Engineered microbes can also be used
to produce the intermediates needed for synthesis of insecticides. However, such chemicals
are not polypeptides, and thus are not a product of a single gene, but are chemicals
produced by multistep biosynthetic pathways. And optimization of the end product
would require characterization and cloning of all the genes involved in the process so that
all the steps in the process are performed in a concerted manner without disturbing the
cell metabolism.
Entomopathogenic Viruses
The family of viruses belonging to Baculoviridae has the greatest potential as biopesticides
because of their specifi city to arthropods (Table 8.1). Narrow host range and slow
action limit the use of entomopathogenic viruses. Genetic engineering provides an effective
means of improving virus activity, and to this end, a variety of recombinant “rapid
action” nuclear polyhedrosis viruses (NPVs) have been developed (Figure 8.1). A major
focus of research on insect viruses involves engineering the viruses to express genes from
other organisms that code for insecticidal proteins, which either kill the insects or disrupt
normal physiological functions. Recombinant baculoviruses expressing specifi c toxins can
be generated rapidly, and applied on a range of crops. Genes encoding neurotoxins have