SUMMARY: Nitrogen starvation and heterocyst development were induced in the cyanobacterium 7120 by growth in nitrogen-free medium or by treatment with the amino acid analogue methionine sulphoximine. During the first 6 h of nitrogen deprivation, amino acid levels and rates of protein synthesis, as measured by the incorporation of [H]leucine, decreased to 50 to 70% of those in ammonia-grown organisms; after this time there was no difference between the rates of protein synthesis in ammonia-grown and nitrogen-starved cultures. The period 4 to 12 h after the onset of starvation was marked by the release of [H]leucine from previously labelled proteins at a rate 6 to 7·5 times that of ammonia-grown organisms. These results are consistent with the idea that nitrogen starvation in cyanobacteria causes a reduction in protein synthesis and leads to the rapid degradation of storage proteins. In rapidly growing 7120, the doubling time for total cell protein was estimated to be 14·9 ± 1·0 h and the half-life was 139 ± 88 h.


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