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Abstract
SUMMARY: Free and membrane-bound ribosomes were isolated from wild-type Neurospora crassa containing the genetic backgrund Em 5297a at 6, 12, 18, 24, 36, 48 and 72 h of growth. Log phase began at 8 h and continued to the end of 24 h, the beginning of the stationary phase. The ratio of free to membrane-bound ribosomes remained relatively constant during log phase. During stationary phase, the ratio of free to bound ribosomes rose sharply because of a decrease in the concentration of bound ribosomes.
Free ribosomes are generally assumed to synthesize intracellular proteins, whereas bound ribosomes synthesize proteins intended for secretion. These changes in membrane-bound ribosomes can be correlated with the specific activity of wallbound invertase (an intracellular enzyme) of the same strain of Neurospora. We suggest that this decrease in membrane-bound ribosomes during stationary phase is of physiological significance and occurs because of a decreasing synthesis of secreted proteins.
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