Please post: RNA in vitro transcription question

[FEI YU]

Hello, does anyone have experience to get high yielding in 
vitro RNA transcription cDNA containing T7 promoter?  My 
problem is whenever I used NTP concentration higher than 6 
mM; it completely inhibits transcription reaction.  I have 
tried to added polyamine and inorganic pyrohosphatase into 
my reaction buffer as suggested by others; it didn't help a 
lot.  I know some commercial kits give very high yielding, 
but they don't sell buffer separately, and all the kits are 
too expensive and contains stuff I don't need, I wish to 
make my own buffer to get high yielding, any thoughts? 
Thanks a lot
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