Ligation problem: Phosphotasing Sac I ends.

[J. Pat Martinez]

I'm having difficulty cloning a 7 kb Sac I fragment into pUC 19.  One
problem has been the phosphotasing of pUC 19/SacI.   I phosphatased the
vector twice with shrimp alkaline phosphotase.  Comparing transformations
of vector without ligase and vector with ligase,  I get thousands of blue
colonies for the vector with ligase.  Self ligation of the vector in the
absence of insert has also been confirmed by gel electrophoresis.  
Apparently the 5' *recessed* phosphates are not being removed completely. 
Any suggestions?

Pat M.