Breakdown and Reassembly of Rat Liver Ergosomes after Administration of Ethionine or Puromycin.

It is now generally recognized that ribosomal aggregates (ergosomes or polysomes) rather than single ribosomes (73 S monomers) are the structures normally engaged in protein synthesis in the cytoplasm of rat liver (l-4) and other mammalian cells (5-13)) as well as in leaves of higher plants and in microorganisms (14-16). The conclusion that each ergosome is held together by one continuous strand of messenger ribonucleic acid is supported by the observations that (a) breakdown of ergosomes into single ribosomes and loss of function follow the addition of traces of pancreatic ribonuclease (1,8, 17), (b) breakdown in vivo follows inhibition of messenger ribonucleic acid synthesis with actinomycin (2, 4, lo), and, most directly, (c) proportionality exists between the molecular weight of messenger ribonucleic acid and the number of ribosomes in the aggregate from which it was isolated (18). Kinetic analysis (3, 11, 12, 16, 19) of the functional state of ergosomes strongly supports the hypothesis (8, 15, 20) of the tape mechanism of protein synthesis. According to this concept, the read-out process by which the genetic message is translated into the colinear (21,22) structure of a growing polypeptide chain requires the movement of mRNAl through a condensing site on each ribosome. The process begins with the successive attachment of single ribosomes to one end of the messenger strand and terminates with their release at the other end, each with its finished polypeptide chain. Thus, during the lifetime of a messenger RNA molecule in viva, the aggregates engaged in protein synthesis are maintained in a steady state by the prompt reattachment of monomers. The spacing of the ribosomes on the mRNA strand, and hence the aggregate size corresponding to an mRNA of a given length, is determined by the rate at which ribosomes move along the messenger (read-out rate) relative to the rate of monomer attachment. Because of the breakdown and assembly of aggregates resulting from mRNA turnover, the steady state size distribution of an intracellular ergosome population would also be dependent upon the rate of mRNA turnover relative to the rate of the read-out process (16). Data published