Biomedical Engineering Reference
In-Depth Information
the Baculovirus family, also called Nuclear Polyhedrosis Virus for reasons which
become clear with a knowledge of their replication cycle, have been isolated from
Lepidoptera, Homoptera (aphids and their relatives) and Diptera. The infectious
cycle passes through a stage where several virus particles are bound together
in a large crystal of protein. This protects the virus particles until the crystal is
ingested by an insect where enzymes in the gut digest this polyhedrin protein
releasing the viruses. These enter the insect's cells where they are uncoated,
make their way into the cell nucleus, and the viral replication cycle commences.
Some 12 hours after initial infection, virus particles are released which spread
the infection to neighbouring cells. By 24 hours post infection, the protective
polyhedrin protein coded for in the viral DNA, is being produced in sufficient
quantities to start assembling the crystal structures. By this time all the insect tis-
sues are suffering severe damage such that at the time of death, the insect is
effectively a mass of virus particles surrounded by the insect's cuticle. This
cadaver is eaten by birds, and consequently may be spread some distance. The
virus is carried intact in the bird's gut protected in the polyhedra, which is resis-
tant to digestion by the enzymes found in avian gut. Polyhedrin protein is made
in enormous quantities in the infected cell, but since its only known function is to
protect the virus in vivo , in the wild, then it is redundant when culturing the virus
in vitro , in the laboratory. This being the case, the coding sequence for the poly-
hedrin protein may be replaced by a 'foreign' gene where under the control of
the polyhedrin promoter it may, depending on the gene being replaced, have a
good chance of being expressed at a very high level. This methodology used
to overexpress proteins, which was described in Chapter 9, has been used with
great success but increasingly in biotechnology fields other than environmental,
notably pharmaceuticals.
Plant/microbe interactions
The microbiology of soils and plant microbe interactions are enormous topics
worthy of the many topics and research papers on the subjects, some of which
are listed in the bibliography. It is not the aim of this section to give a detailed
account but simply an introduction to the complexity of plant-microbe interac-
tions in the hope of illustrating that 'no man is an island': disturbance of these
interactions has its consequences. Although the term 'plant' includes all plant
forms from trees to algae, the current discussion addresses interactions between
higher plants and micro organisms. Such interactions fall into two basic cate-
gories: the first category being those involving microbes external to the plant,
such as soil bacteria and soil fungi. The second being microbes internal to the
plant, which include endophytic bacteria such as those involved in nitrogen fix-
ation, internal fungi and plant pathogens examples of which are Agrobacterium
plasmodium and Agrobacterium tumefaciens (Greene and Zambryski, 1993). The
latter is now used extensively to introduce 'foreign' genes into plants as described
in Chapter 9. The associations may therefore involve bacteria, fungi or viruses
and in some cases, some quite complex interactions involving three or four
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