May 12, 2004 — Scientists at the John Innes Centre (JIC), Norwich (1) have today
reported that a very successful antibiotic, which is harmless to humans
but lethal to most bacteria, also kills plants. They have found that an
enzyme, which is an important target for several families of
antibiotics and was thought to exist only in bacteria, is also present
in plants. The discovery sheds further light on plant evolution and
highlights a potential area for development of new herbicides, while it
has no significance with regard to the medical use and efficacy of the
antibiotic. The discovery is reported online and in the latest volume
of the international scientific journal Proceedings of the National
Academy of Sciences, USA.
"Our interest is in the structure of DNA
and in particular in an enzyme called DNA gyrase(2), which is crucial
for maintaining the structure of the DNA molecule in bacteria" says
Professor Tony Maxwell (Head of Biological Chemistry at JIC). "The
antibiotic ciprofloxacin (Cipro), made famous in 2001 during the
anthrax attacks in the US, targets DNA gyrase which, until now, we
thought was a bacterial ‘Achilles heel’ because it had only been found
in bacteria. However, we have now discovered that plants are sensitive
to Cipro and that is because DNA gyrase is also important in plants".
Working
with the common weed Thale Cress (Arabidopsis thaliana) Dr. Melisa
Wall, a member of Prof. Maxwell’s team at JIC, found DNA gyrase in both
the chloroplasts and mitochondria in plant cells. These tiny structures
(called organelles) are responsible for carrying out photosynthesis and
respiration respectively (3). Scientists think that organelles
originated from bacteria that were able to live in plant cells and over
evolutionary time they eventually became highly specialised to perform
particular functions for the cells they were living in. The fact that
the organelles still use DNA gyrase is an echo of their distant past as
free-living bacteria.
Plants manage their DNA in a very different
way from bacteria and have no use for DNA gyrase. Yet the relationship
between the organelles and the plant cell has become so sophisticated
and intimate that the genes encoding DNA gyrase are actually present in
the DNA molecule of the plant nucleus, not in the organelles
themselves. When the DNA gyrase enzyme is produced in the plant cell it
is tagged with a fragment of protein that directs it to the appropriate
organelle, where it carries out its function of DNA maintenance.
"The
discovery that Cipro can kill plants raises the possibility of
developing new herbicides based on the structure of Cipro and other
drugs that target DNA gyrase" says Dr Wall. "Cipro kills bacteria very
effectively but has low animal toxicity. We are now studying the DNA
gyrases found in plants and bacteria to see if we can exploit
differences between them to selectively kill plants while leaving other
organisms, especially bacteria, unaffected".
Source : John Innes Centre