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What is the "p" in CpG?

Nicolas Le Novere lenov at pasteur.fr
Tue Jun 17 06:31:11 EST 1997

In article <bregeon-1706971048590001 at ravel.ijm.jussieu.fr>,
Bregeon Damien <bregeon at ijm.jussieu.fr> wrote:
>In article <5o3fqs$8vd$1 at vixen.cso.uiuc.edu>, dneece at ux1.cso.uiuc.edu
>(David Neece) wrote:

An interesting point about CpG islands. The "school" explanation of their 
rarity is the methylation. The methylases act specifically on these sites.
The methylated C is easely transformed by deamination into T. The increased
mutations frequency would be counterselected. 

However, another fact is the thermodynamic behviour of CpG. Indeed this 
"Crick's pair" possesses the highest deltaH of all, for DNA homoduplexes 
(Breslauer et al (1986) PNAS 83 : 3746-3750) but mostly for DNA/RNA
heteroduplexes (Sugimoto et al (1995) Biochemistry 34 : 11211-11216).  
The Tm being increased (Wetmur (1991) Crit Rev Biochem Mol Biol 26 : 227-259),
some problems could occur during transcription. There would be a 
counterselection of the CpG but the action of methylases would be the 
consequence of the CpG rarity (for specificity reasons) and not the cause.

Some idea about that?

Nicolas Le Novere
INSTITUT PASTEUR,Neurobiologie Moleculaire,25, rue du DR ROUX, 75O15 PARIS, FRANCE
Ph: 33-1-45-68-88-44,Fax: 33-1-45-68-88-36,lenov at pasteur.fr

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