Biogenesis of 50 s particles in exponentially growing Escherichia coli.

Abstract

Abstract Synthesis and assembly of structural proteins into 50 s ribosomal subunits were investigated in exponentially growing Escherichia coli with a doubling time of 120 minutes. The study was designed to determine the rate at which pulse-labeled ribosomal proteins appeared on the 50 s subunit. Examination of the total ribosomal fraction for label following a one-minute pulse with [14C]leucine and subsequent chase with unlabeled leucine revealed that all of the labeled ribosomal protein appeared in the ribosomal fraction only after a chase period of approximately 30 minutes. A study of individual 50 s ribosomal proteins fractionated by polyacrylamide gel electrophoresis indicated that the rate of appearance of labeled proteins varied. The average half-life time of labeled ribosomal proteins prior to their appearance on 50 s subunits was found to be approximately five to six minutes. The half-life times of individual 50 s ribosomal proteins varied between two and eleven minutes. From the rate of appearance of labeled protein on the 50 s subunit, the amount of 50 s ribosomal protein not yet associated with mature 50 s subunits was estimated to be about 5% of the total cellular 50 s ribosomal protein. The amount of individual 50 s ribosomal proteins not associated with mature subunits varied from approximately 2 to 9%. Examination of the labeled proteins on the 50 s subunit after a 15-second pulse with [14C]leucine and no further incubation suggests that there exist at least three groups of 50 s ribosomal proteins added at different rates, consistent with the contention that proteins are added during the maturation of ribosomal subunits forming precursor particles and finally the mature 50 s subunit.