Identification of a Precursor Pool of Ribosome Protein inEscherichia coli

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Abstract
Antibodies prepared against proteins from 50S ribosomes of E. coli also reacted with the supernatant proteins of a cell-free extract of E. coli which was ribosome-free. A reaction of immunological identity (Ouchterlony tests) was demonstrated for 1 of these supernantant proteins and 1 protein found in 50S ribosomes. Isotope experiments involving a shift from 14C-leucine medium to 12C-ieucine medium showed that these proteins are not formed by breakdown of ribosomes during the preparation of cell-free extracts, but instead represent a pool of ribosome protein which is utilized during growth. In shift experiments from 14C-leucine to 12C-leucine medium, the kinetics of disappearance of labeled supernatant ribosome proteins (as measured by reaction with antibody) indicated that 1/2 the pool is depleted in 0.1 generation time at 37 C in glucose-salts medium. The pool was also depleted under conditions of amino-acid starvation of a "relaxed" strain which accumulated "relaxed" particles. Most, if not all, of the protein present in "relaxed" particles was derived from the pool. The pool represented about 3-4% of the total soluble proteins in the ribosome-free supernatant fluid of an E. coli extract.