A STUDY OF THE NASCENT POLYPEPTIDES SYNTHESIZED ON THE FREE POLYRIBOSOMES OF RAT BRAIN IN VIVO 1

The free polyribosomes of the cerebral cortex of the immature rat (12-14 days old) were exposed to very low concentrations of trypsin at 0°C and for very brief periods of time and the conditions under which their breakdown to smaller aggregates occurs were determined. Trypsin also caused the release of nascent, radioactive polypeptides from polyribosomes prelabelled with [ 14 C]amino acids in vivo. An examination of the kinetics of release of the nascent chains by trypsin revealed that it was dependent on the concentration of trypsin as well as on the duration of incubation in the presence of trypsin. The influence of the nature of the [ 14 C]amino acid used as precursor of the nascent polypeptides and of the duration of the radioactive pulse in vivo was also determined. The radioactivity associated with polyribosomes as a result of the brief radioactive pulses administered (2 to 10 minutes) was incompletely removed even after the ribosomes were dissociated into subunits by EDTA. These findings suggest that the assembly of the cerebral ribosome in vivo must be a very rapid process, particularly in the immature animal. The nature of the nascent, radioactive polypeptides was studied by disc gel and high voltage electrophoresis and by thin-layer and column chromatography. Evidence was obtained that a rather limited number of qualitatively different molecules resides on the polyribosomes at any given moment.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65454/1/j.1471-4159.1971.tb00223.x.pd