The Fractionation of Transfer Factors in the Presence of a Proteolytic Inhibitor

By use of a proteolytic inhibitor, phenylmethylsulfonylfluoride, we have been able to fractionate from the ribosomefree cell sap of Escherichia coli a factor that is able to catalyze polyphenylalanine synthesis with purified ribosomes. In the absence of the inhibitor, we were unable to recover this factor, but instead obtained numerous substances none of which were active alone with purified ribosomes and which were similar to the previously described transfer factors "T" and "G". During purification, the inhibitor-stabilized factor develops a requirement for a heat-stable protein which cannot be replaced by either T or G.