Recrod thermophile, with references

J.R. Pelmont Jean.Pelmont at wanadoo.fr
Wed Apr 8 04:57:43 EST 1998


Brian Davis <quark at umich.edu> wrote (écrivait) :


> .............
> > The double helix of DNA gets denatured,... This occurs between 70 
> > and 110°C according to the base composition.
> 
>    Interesting - what differences in bases allow this range of
> temperatures? More hydrogen bonding?

Hydrogen bonding is one factor, together with base stacking. G+C rich
DNA stretches are more heat stable because 3 H bonds are established
between guanine and cytosine instead of 2 between adenine and thymine. A
DNA with 40 p.cent (common in mammalian genomes) has an average melting
temperature at 87°C, instead of 95° if the G+C was 60 p.cent. But these
values correspond to DNA solutions in standards buffers. It can be
compared to a change of state, but because the DNA sequence and
composition are not uniform along the double-strand helix, it is not
abrupt at a given temperature as in crystal melting. Therefore the
so-called melting temperature of DNA is the temperature when about half
of DNA is denatured.

The melting of DNA is deeply affected by the medium : bound proteins,
organic solvents, ... For instance in Molecular Biology experiments
dealing with hybridization of nucleic strands and oligonucleotide
probes, the melting temperature can be controlled by adding salts or
organic compounds (formamide). The melting of DNA within living cells is
of course a different matter, because DNA is bound and patrolled by many
different structural and regulatory proteins in a surrounding medium
which is highly concentrated in biomolecules and certainly very
different from plain water. This problem is accute in hyperthermophiles,
and has not been really solved. They have a high G+C content, but this
does not explain fully the property. Protection of DNA at high
temperatures certainly requires specialized molecular devices. For
instance certain proteins have been found to be extremely abundant in in
hyperthermophiles studied.

One of these proteins is glyceraldehyde 3P-dehydrogenase. Besides its
normal function in central metabolism, this peculiar protein is able to
bind to monocatenar DNA and to do other things. This may explain why
this enzyme and isoforms have evoluted so slowly in life history, with
considerable homology of sequence between unrelated species. Apparently
because any mutation for this strategic tool is likely to disturb an
important function.

But DNA is not the sole problem. Denaturation of protein by heat is an
other factor. Usually hydrogen bonds become labile at heat, while
hydrophobic interactions are stronger. Thermostable proteins show only
some small differences in sequence and structure when compared to
homologous proteins of other species. Disulfide bonds and metal ions
(Zn, Ca, Mg) may also enhance thermostability. This is a really hot
problem for research, because so many firms and laboratories are eager
to produce stable enzymes for industrial purposes and try to mutate
defined residues in the sequence of various microbial or fungal enzymes
such as proteases, amylases or others.

One accomplishment of heat-stable protein research has been the
discovery of the Taq DNA-polymerase of Thermus aquaticus (Cetus Co) in
the eighties, the starting material for the PCR method in DNA studies,
and a long jump forward for science !

Most interesting are studies of the so-called heat-shock proteins in non
thermophilic organisms such as E. coli. They have initiated a large
field of research and may help also to understand better how life
resists at elevated temperature. The so-called molecular chaperones or
chaperonines are peculiar heat-shock proteins, the synthesis of which is
induced by thermal stress. These proteins are necessary for cell
viability in any conditions, because they are believed to help the
correct folding of newly translated product and to facilitate the
assembly of quaternary structures. They are complex proteins also
allowing polypeptides to renature or refold properly, using ATP
hydrolysis as a source of energy. Similar proteins with ATPase activity
have been found to accumulate in some hyperthermophiles. The most famous
chaperones are GroELS and DnaK of E. coli (which is not thermophilic :
it is fast destroyed above 47°C). The detailed X ray structure
of GroELS has been published recently : Xu Z, Horwich AL, Sigler PB
(1997) Nature 388, 741-749: The crystal structure of the asymmetric
GroEL-GroES-(ADP)7 chaperonin complex.
> ..................      
>.................. Based on "Five Kingdoms" by Margulis & Schwarts,
>these things (multicellular animals, about 0.5 mm across) survive
>temperatures from -270 deg-C to 151 deg-C, radiation about 1000 times
>the human lethal limit, dessication, and can form a spore- or cyst-like
>structure called a Tun that can survive for around 100 years. I'm
>*amazed* it can survive these extremes.

You have mentionned in your post the Tardigrads. I know nothing on this
subject, which sounds extremely interesting. Certainly many species have
developed survival forms that do not replicate and have acquired some
enormous resistance not owed by multiplying vegetative cells or usual
organisms. Bacterial spores we believed to be the most resistant forms
to heat and storage so far, but surprising new facts are to be expected.
As a matter of fact many species have latent resistant stages, such as
cyanobacteria, yeasts, molds, worms, daphnia, rotifers and others. The
Tardigrads seem to be very efficient in this respect. I remember I have
seen small strange-looking shrimps in the australian desert. They
develop in small ponds and flakes of water on sandstone rocks after a
rain, grow and mate within a few weeks. Once dessicated the eggs resist
to the strong sunshine heat and are carried by wind, being able to keep
viability after months or more.  This has been explained to me by an
australian companion. I heard since that similar shrimps can be seen in
Sahara. Unfortunately, I have no scientific data on these animals.
 
Certainly many new forms remain to be discovered, that are extremely
resistant to various conditions of stress. The bacterium Micrococcus
radiodurans develops in the cooling systems of nuclear plants,
apparently because they have very efficient DNA repair systems. Recently
new bacterial forms have been found that require high pressure to
develop, as found in cold waters at high ocean depth. Is it not
wonderfull the power of life ?

Sorry for this long post, I took pleasure to participate to this
discussion (and we have a few days of holiday here). Hoping you were
interested, sincerely.  


-- 
J. Pelmont, Biochimie
Univ. Grenoble I, jean.pelmont at ujf-grenoble.fr



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