The enzyme used to seal the gene permanently into the plasmid is

Explanation

Sticky ends pair by hydrogen bonds only, which is not strong enough to hold, so ligase forms the covalent phosphodiester bonds in the sugar phosphate backbone and completes the recombinant molecule. Restriction enzymes do the opposite job of cutting, which is why the two are always taught as a pair. Ligase is the same enzyme that joins Okazaki fragments during normal DNA replication.