Is there a protein ligase?

Results obtained from experiments dealing with mammalian, bacterial, phage and mitochondrial protein biosynthesis as well as certain enzymatically performed amino acid replacement studies on Kunitz trypsin inhibitor strongly suggest that protein ligation may be occuring in vivo. Amino acid substitution experiments prove the reversibility of endopeptidase reactions, and protein ligation is the reverse of endopeptidase reaction. These experiments are discussed in detail and the suggestion is made that ligation may also be useful in the repair of certain essential proteins which may become damaged.