I am a bacteial geneticist, and my interests concern the regulation of
DNA topology, and its effects upon gene expression. In bacteria,
numerous genes are coupled to promoters that are sensitive to the
levels of DNA supercoiling. This provides an extremely powerful
mechanism for regulating a large number of unrelated genes with very
diverse functions, around the chromosome. DNA supercoiling is affected
by changes in environmental conditions. Pathogenic bacteria have
exploited this by wiring up their virulence circuits to this system.
This ensures that they only express their virulence determinants
within the host, and in the correct site within the host.
Supercoiling of DNA is primarily determined by the two competing
enzymes, topoisomerase I and DNA gyrase. These 2 enzymes establish an
equilibrium that is optimal for the ambient conditions. However, if
there is a sudden change in environmental conditions such as pH,
osmolarity, aerobicity, temperature etc, these 2 enzymes are required
for changing the levels of supercoiling to enable the bacterium to
regulate its metabolism and physiology accordingly.
The histone-like DNA binding protein, H-NS, has been shown to modulate
DNA supercoiling in response to environmental stimuli, but nobody has
in my opinion, satisfactorily explained how this is done. In hns
deficient mutants, their DNA supercoiling levels do not change in
response to environmental stimuli. Nobody has been able to demonstrate
satisfactorily, that H-NS acts as a topoisomerase in vitro (unlike
gyrase or topoisomerase I). H-NS is known to bind to curved DNA, often
associated with environmentally regulated promoters. It acts as a
transcriptional repressor upon binding to these promoters. Moreover,
as the levels of H-NS increase, so does the level of supercoiling.
I have a model for how H-NS might be acting to regulate DNA
supercoiling, but I would like others to offer their own ideas as to
whether I'm talking rubbish or whether there might be some possibility
of its validity. The model goes thus:
H-NS levels are regulated by the interaction of regulatory proteins
with the hns promoter (eg the cold shock protein, CspA). H-NS binds
to the promoter of gyrA and/or topA (the genes for gyrase A subunit
and topoisomerase I, respectively), but its ability to bind to the
promoters of these genes is dependent upon the level of supercoiling
and promoter curvature. It has already been shown using the hns
promoter, that the ability of H-NS to bind to its own promoter is
supercoiling dependent. Thus, upon binding to the gyrA or topA
promoters, their expression can be attenuated, resulting in the
enzyme activity of the competing enzyme pushing the equilibrium one
way or the other.
This model obviously needs considerable refinement, but I hope that it
provides a nucleus for discussion. Any ideas?????
*****************************************
* Dr Martin Goldberg, *
* Dept. of Microbiology and Immunology, *
* University of Leicester, *
* University Road, *
* LEICESTER. *
* LE1 9HN *
* UK *
* Tel. +44 (0)116-252 3017 *
* Fax. +44 (0)116-252 5030 *
* E-mail mdg at le.ac.uk *
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