Formation and life cycle of ribosomal subunits, mono- and polyribosomes in Bacillus licheniformis

Abstract

The kinetics of formation and the metabolic fate of the ribosomal particles in exponentially growing cells of Bacillus licheniformis was studied by the pulse-chase technique. It was found that the incorporation of labeled RNA and protein precursors into free ribosomal subunits, which are the first ribosomal particles to become labeled, is completed in about 10 min. The pulse-labeled ribosomal protein appeared on the subunits a few minutes after the labeled ribosomal RNA. This delay is most probably due to a small cellular pool of free ribosomal protein which was found to exist in B. licheniformis. The ribosomes, once formed, were found to be very stable; no breakdown of labeled ribosomes was observed during four cell generations. The newly formed subunits were found to recycle through the polysomal complex not very rapidly, with a striking periodicity. One complete cycle required about 10 min. When exponentially growing cells of B. licheniformis are lysed and analyzed in ionic conditions mimicking those of the intracellular fluid, a relatively large proportion of 70S particles is found. The kinetic data obtained show that at least a part of these particles must be considered as true intermediates in the ribosome cycle.