2. FEATURES
Figure 1
The burning of fossil fuels is a major source of pollution (illustration courtesy of Photodisc).
bacteria that are capable of desulfurizing model com- the black root rot disease of tomato, which is caused by
pounds, such as those found in fossil fuels1, were inves- the fungus Thielaviopsis basicola4. Hydrogen cyanide was
tigated. It is thought that any large-scale process must shown to be biosynthesized from glycine by P fluorescens
.
ideally be carried out at temperatures approaching the expressing the hcnABC gene cluster. Mutants that failed
upper range tolerated by microorganisms. to fully express the hcnABC genes were partially
impaired in their ability to suppress the growth of
Alternative energy T. basicola. This could potentially form the basis of a
Of even greater potential application would be a new biological control mechanism for hard-to-manage
biotechnological process that would eliminate the need plant fungal pathogens.
for current high-volume fossil-fuel usage by providing
an alternative energy source. For example, cars that use Hazardous wastes
hydrogen fuel are currently under development. One long-standing environmental use of microor-
Hydrogen is an ideal non-polluting energy source as it ganisms is for the remediation of hazardous wastes, and
burns to produce water as the only product. However, new approaches are emerging to treat the most drastic
for widespread acceptance of this new technology, it mixed waste problem. One of the legacies of the Cold
will be necessary to generate hydrogen cheaply, for War remains: the nuclear-weapons production facilities
example, using the energy freely available from sun- and their decades of spent wastes. These contain a
light. One recent study sought to link the photosystem mixture of organic wastes, heavy metals and high-
of bacteria directly to a reversible hydrogenase to trans- energy radionuclides. In the USA alone, the Department
form the energy from sunlight into biogenic hydro- of Energy has estimated the cost of cleaning up these
gen2. The critical issue here is the level of efficiency in sites to be as high as US$250 billion. Innovative tech-
capturing photosynthetic-electron flux; it must approach niques are sought, but it is appreciated that the vast
the chemical-energy efficiency of natural photosyn- majority of microorganisms would not survive the radi-
thetic organisms to make the process economically ation fluxes present at these sites. In this context, an
feasible. This has not yet been attained, but the goal is extremely radiation-resistant bacterium, Deinococcus
well worth greater effort. radiodurans, is being genetically engineered with
biodegradation genes to render it suitable for the
Chemical pesticides treatment of mixed wastes5. The broad-specificity
Another area of environmental protection is the Pseudomonas enzyme, toluene dioxygenase, has been
application of biological controls to supplant the use of cloned, expressed and shown to be active in D. radio-
chemical pesticides. There are commercial products durans, even in the presence of high fluxes of ionizing
that are currently on the market, for example, Bacillus radiation.
thuringiensis has been used to selectively control certain
insect pests. In this case, the whole organism is mar- Novel biocatalysis and biomaterials
keted. Another biotechnology market is in transgenic The hydrocarbon-dihydroxylating dioxygenases of
plants that biosynthesize the B. thuringiensis insecticidal the type cloned into D. radiodurans have been studied
protein in situ. This has proven to be highly effective, for 20 years and interest is increasing to exploit the
for example, against the cotton bollworm3. enzyme-generated chiral centers for use in synthesiz-
The killing action of B. thuringiensis parasporal pro- ing complex chiral molecules6. Only recently, however,
tein against insects has been known for decades. More has a model of the active site been available, based on
recently, it has been discovered that certain Pseudomonas the X-ray structure derived for naphthalene dioxygen-
strains enzymatically generate hydrogen cyanide and ase7. This should spur interest in protein and metabolic
this trait is linked to the organism’s ability to suppress engineering using this broad class of dioxygenases.
20 TIBTECH JANUARY 2000 (Vol. 18)
3. FEATURES
Many important commercial organic chemicals and
synthetic intermediates contain halogen substituents.
Halogens are not typically thought of as being biologi-
cally relevant functional groups, but they are found in
naturally occurring antibiotics and other natural prod-
ucts. The biosynthesis of these compounds has largely
been attributed to enzymes known as haloperoxidases,
but many natural halogenated compounds could not be
explained as coming from a haloperoxidase-type reaction
mechanism. Recently, a new mechanism for biohalo-
genation has been proposed and this also explains the
occurrence of novel fluorinated compounds that derive
from biological systems8.
When important new biochemical activities are
detected, it is often of interest to modify a natural
enzyme to achieve a desired substrate specificity, heat
stability or higher activity. Rational protein design is
still a very difficult task if one wishes to obtain an
‘improved’ enzyme. In this context, biologists are tak-
ing a cue from the chemists and expanding the use of
combinatorial methods to modify enzymes. One exam-
ple of this approach generates genetic diversity by a pro-
cess known as DNA shuffling. DNA shuffling produces
many enzyme variants that can be screened or put
through a biological selection procedure to obtain one
or more desired traits. Recently, it has been found to Figure 2
be powerful to start the DNA-shuffling process with Plants as potential expression systems for the production of desirable
natural genetic diversity9. The idea is to use several proteins (illustration courtesy of Photodisc).
natural variant genes because these divergent sequences
have already been selected for their ability to produce and plant biology, will be spurred on by widespread
proteins that fold correctly and have reasonable enzyme genome sequencing. The discovery of new gene prod-
activity, thus enhancing one’s overall success in obtaining ucts will continue to lead to new commercial products.
the improved enzyme.
References
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Plants (Fig. 2) are potentially marvelous expression targeted biodesulfurization. Appl. Environ. Microbiol. 63, 3164–3169
systems for the production of desirable gene products, 2 McTavish, H. (1998) Hydrogen evolution by direct electron transfer
such as natural plant products and foreign proteins. It from photosystem I to hydrogenases. J. Biochem. 123, 644–649
may be possible to use the energy of sunlight to drive 3 Estruch, J.J. et al. (1997) Transgenic plants: an emerging approach
to pest control. Nat. Biotechnol. 15, 137–141
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order to become more widespread, this will require a encoding hydrogen cyanide synthase and anaerobic regulation by
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recently, Arioli et al.10 reported new insights into the 5 Lange, C.C. et al. (1998) Engineering a recombinant Deinococcus
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plant-model system. waste environments. Nat. Biotechnol. 16, 929–933
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reaction mechanism of PHA synthase13. Site-directed 10 Arioli, T. et al. (1998) Molecular analysis of cellulose biosynthesis in
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