Duerre, John A. (University of North Dakota, Grand Forks). Incorporation of S35 into ribosomal proteins of Escherichia coli. J. Bacteriol. 88:130–136. 1964.—The rate of incorporation of S35 was followed in exponentially growing cultures of Escherichia coli. Within a few seconds, the isotope was incorporated into proteins, appearing first in or on the ribosomal particles. Ribosomal proteins from these particles were fractionated into molecular weight classes by gel filtration. The radioactivity from S35O4= appeared in the proteins of large molecular weight at a faster rate than in those of small molecular weight. This radioactivity could be displaced from the large molecular weight class with nonradioactive sulfate more rapidly than from the small class. The large molecular weight protein could also be distinguished from the small molecular weight proteins on basis of their higher sulfur content, which corresponds to that of soluble proteins.