Copies of protein S6 in Escherichia coli ribosomes

A reinvestigation of the copy number of protein S6 in Escherichia coli ribosomes shows that there is only one copy of S6 in 30 S subunits and in 70 S ribosomes. The previously reported higher value (Subramanian, A. R. (1980) J. Biol. Chem. 255, 6941-6946) is shown to arise from the presence in the usual ribosome preparations of a protein which co-electrophoreses with S6 but does not react with S6 antiserum. This protein is removed when ribosomes are purified by passage through a sucrose gradient.