80 S ribosomes inAcetabularia major distribution and transportation within the cell

The 80 S ribosomes of the unicellular green algaAcetabularia are evenly distributed throughout the cell. In pulse-labeling experiments it has been ascertained that newly-formed ribosomes are entirely restricted to the basal part of the cell. They first appear as free polyribosomes and become largely membrane-bound within 5 to 7 days. Labeled ribosomes are not observed in the apical half of the cell until ten days after their formation. Two or three weeks later labeled ribosomes are primarily concentrated in the apical half of the cell. These results favor a directed mechanism of transportation. The half-life of the 80 S ribosomal RNA was estimated to be about 80 days. On the average each cell yields 200 ng of 80 S ribosomes, an amount which is synthesized within a period of 2 months. From this rate of synthesis it can be concluded that the genes for the RNA of the 80 S ribosomes have a redundancy of more than 10,000.