Help! Three way ligation including blunt-end oligo - (Feb/17/2005 )

There are three fragments in my ligation, one is the EcoRV(blunt end)-NcoI fragment of plasmid backbone. Another is an oligo(~40bp) containing EcoRV(blunt end) and KpnI site on two side respectively, and the other is pcr fragment cut by KpnI and NcoI. I have tried many many times of ligation, but all the clones i picked out were like the wild type plamid (seemed a bit smaller). Do I need to phosphotase the plasmid backbone? Or should I increase the amount of each fragment and T4-ligase? Please help me...

how long are you doing the ligation?

Try prolonging it. I have never done a 3-way ligation but someone I know did an overnight ligation at 16C, then the whole of next day at 4C.

what are your molar ratios? I would try to increase the amount of your oligo because of the blunt end ligation.