LONG PCR. HELP

[Blanca Barquera]

I have tried to amplify a 15 kb fragment from chromosomal DNA 
from Paracoccus denitrifcans, using "Expand long template PCR 
system from Boehringer. I have tried several conditions and  I 
have gotten nothing or a big product (20kb) unrelated with what I 
want. I have used DMSO to denature DNA also unsucesfully. My DNA 
is rich in G+C.I have changed buffers, template concentration (50 
to 250 ng. Mg concentration and amount of enzyme. I do not know 
what to change.My primers have worked to get smaller fragments.
Some help would be appreciated. Thank you
Blanca Barquera
University of Helsinki