Purification and Best Department of Medical Research, University of Toronto, Ont., Canada.

Factor EF-P is a nonribosomal (soluble) protein of Escherichia coli that stimulates peptide bond synthesis when certain aminoacyl-tRNA analogues are used. The purification of this protein to apparent homogeneity is described here. EF-P has a molecular weight of about 21 000, a Stokes radius of 27 A (1A = 0.1 nm), and a frictional coefficient of 1.48, suggesting an asymmetric structure. By this and a number of other criteria, EF-P is a new factor that controls peptide bond formation during protein biosynthesis.