Tn5cos: a transposon for restriction mapping of large plasmids using phage lambda terminase.

A method for the rapid restriction mapping of large plasmids has been developed. A 400-bp fragment of phage lambda DNA containing the cos region has been inserted into Tn5. After in vivo transposition of this Tn5cos element into the plasmid of choice, the plasmid is isolated and linearized at its cos site with phage lambda terminase (Ter). Such Ter linearization was about 70% efficient. After partial digestion of the linear molecules with the appropriate restriction enzyme, the products are selectively labelled at the right or left cohesive phage lambda DNA termini by hybridization with digoxygenin (DIG)-11-dUTP-labelled (using terminal transferase) oligodeoxyribonucleotides complementary to the single-stranded cos ends. After pulsed field gel electrophoresis, the labelled fragments are visualized in the dried gel using a DIG-detection kit. The restriction map can be directly determined from the 'ladder' of partial digestion products.