SUMMARY: Following transfer to medium lacking a nitrogen source, cells of PCC 6803 continued to divide, giving a doubling of cell number after 40 h. Phycocyanin degradation commenced immediately after the transfer, with a rapid phase lasting 5 h in which 50% of the phycocyanin disappeared and a slow second phase in which the phycocyanin content decreased to 10% of its initial value by 24 h. In the presence of glucose, a utilizable carbon source for this facultatively heterotrophic cyanobacterium, phycocyanin was degraded initially at a rate 60% of that observed in the absence of the sugar and proteolysis was almost completely inhibited after 6 h when only 27% of the phycocyanin had been lost; cell division ceased at this time. Photosynthetic O evolution decreased rapidly in the presence of glucose and the cells consumed O in the light after 7 h, indicative of a switch to oxidative metabolism; net O uptake in the absence of glucose occurred only after approximately 25 h. Inhibition of phycocyanin degradation by glucose required metabolism of the sugar, probably via the oxidative pentose phosphate cycle, and appeared to result from irreversible inactivation of the protease.


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