Evaluation of minimal oligonucleotide substrate requirements for T4 DNA ligase. (a) Schematic diagram of an immobilized DNA strand used in ligation assays and DNA construction. M-270 Dynabeads (Invitrogen) are attached through a streptavidin-biotin linkage to the 5' end of a double stranded DNA. The free end is designed with a variable 5' overhang, complementary to labeled oligonucleotides used in ligation. An additional BbsI restriction site and a forward primer site are included in the case of DNA construction. (b) Increasing concentrations of 5'-phosphorylated, 3'-fluorescently labeled oligonucleotide are ligated to 5 pmoles of immobilized dsDNA with a complementary overhang. Reactions were performed for one hour at 16°C and washed with TE to remove unligated substrate. Successful ligation kinetics are observed at the 5-bp duplex length (▲), but no significant ligation occurs at lengths of 4-bp (■) or 3-bp (◆).