The Collaborative International Dictionary of English v.0.48:

ligase \ligase\ n. (Biochem.)
   An enzyme which catalyzes creation of a covalent bond between
   two substrates, resulting in a larger product which is a
   combination of the two starting materials; especially, DNA
   ligase, an enzyme which creates a phosphate bond between 5'
   and 3' ends of a DNA chain, resulting in a longer chain of
   DNA. DNA ligase is important in normal biosynthesis of DNA in
   the replication cycle of cells.
   [PJC]