Hydration dependent polysome-monosome interconversion in the germinating castor bean endosperm

Castor bean seeds germinated for 40 br under a condition of unlimited water availability were placed for 1 to 5 days in a condition such as to interrupt any further water uptake. The following phenomena were observed: a) the rate of synthesis of glucose-6-phosphate, phosphogluconate and NADP isocitrate dehydrogenases progressively decreased, then completely stopped; b) polysomes disappeared while monosomes correspondingly increased; c) the endogenous capacity of isolated ribosomes to incorporate amino acid into proteins fell to zero, while in the presence of poly-U phenylalanine incorporation remained practically constant. When the seeds were placed again in contact with water, the enzyme activities began to increase again; polyribosomes were reformed both from old ribosomes preexisting in the treated endosperms and from new synthesized ribosomes; the endogenous capacity of the ribosomes to perform protein synthesis (on an RNA basis) regained their original levels. These data suggest that some factor affecting mRNA or its interaction with ribosomes, rather than a modification of ribosomal structure is responsible for the stoppage of protein synthesis caused by interrupting water uptake.