Abstract
Posterior silkglands of the silkworm were collected from larvae on the 4 th to 6 th day of the fifth instar and after incubation in vitro with glycine-C14, incorporation of radioactivity into fibroin in subcellular fractions was studied. The greatest and most rapid incor-poration was observed in the supernatant (105 S) after centrifugation at 105, 000×g for 60 minutes. After DOC treatment, the soluble part (M-S) of fraction M (precipitate after centrifugation at 105, 000×g for 60 minutes) showed an incorporation curve similar to that of 105S, but the incorporation to the DOC-insoluble part (M-P) of fraction M was less and similar to that of the DOC-soluble part (LM-S) of fraction LM (precipitate after centrifugation at 20, 000×g for 30 minutes). The rate of labelling of fibroin in the insoluble fraction (LM-P) after treatment of fraction LM with DOC was, as it was also the case with fraction CD, almost linear during the reaction period.
After 15 minutes' incubation, the specific radioactivity of fibroin in fraction R was much higher than that in fraction S, which was obtained by further centrifugation of fraction 105S for 180 minutes at 105, 000×g.
Sucrose density-gradient sedimentation analysis of fractions R and M indicated that fraction R mainly consisted of free ribosomes, while fraction M contained large, hetero-geneous particles. In fraction R, the radio-activity was found in the ribosomal peak. There was unexpectedly low labelling of fraction M.
The effect of ribosomes being in a free state on fibroin synthesis was discussed.
