Summary: In , protein synthesis continued during dark periods at 80% of the rate observed in the light. Since nitrogen fixation ceases in the dark, this implies that fixed nitrogen accumulates during the light periods to supply amino acids for protein synthesis in the dark. Measurements of cyanophycin and the distribution of nitrogen in subcellular fractions indicated that cyanophycin does not represent a significant proportion of total cell nitrogen, nor does its concentration vary across the light/dark cycle. Similar results were obtained with ; there was no evidence that cyanophycin accumulated during the dark to support protein synthesis in light. Cyanophycin does not, therefore, appear to serve as a dynamic temporary storage form of newly fixed nitrogen in the integration of light and dark metabolism. We also investigated an immunological approach to the measurement of cyanophycin and its turnover. Cyanophycin is immunogenic, and in the pure state could be assayed by radioimmunoassay, which had greater sensitivity than traditional assay procedures. The insolubility of cyanophycin at neutral pH, however, prevented the successful development of methods for quantifying cyanophycin turnover in cell extracts.


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