sentence: Purified Ubc13 ( 0.2 µ M ) , Uev1 ( 0.2 µ M ) , and E1 ( 0.1 µ M , Boston Biochem ) were incubated in 50 mM Tris-HCl , 5 mM MgCl , 2 mM ATP and 0.5 mM DTT , with either 117 µ M wild-type ubiquitin ( Biomol ) , 6 µ M UbK48_only or 6 µ M UbK63_only ( Boston Biochem ) .