Phycobiliprotein degradation can be initiated in cultures of the cyanobacterium Anabaena by removal of combined nitrogen from the medium. Certain strains of Anabaena differentiate cells specialized for aerobic nitrogen fixation (heterocysts) under such conditions. We describe here a procedure for the preparation of extracts from heterocysts or vegetative cells that contain an activity capable of degrading only the phycobiliproteins in a mixture of soluble Anabaena proteins in vitro. This activity increased under nitrogen starvation conditions or in ammonia-replete cultures treated with the glutamine synthetase inhibitor methionine sulfoximine. The increase in activity induced by nitrogen starvation was prevented by chloramphenicol or by carbon starvation. Under all these conditions, phycobiliprotein degradative activity assayed in vitro was correlated with the loss of phycobiliprotein absorbance in vivo. Finally, starvation of a met auxotroph of Anabaena for methionine (in the presence of ammonia) did not induce phycobiliprotein degradation in vivo or the increase in proteinase activity. Together with direct measurements of ppGpp, these results indicate that proteolysis in Anabaena is not controlled by compounds associated with the stringent response in Escherichia coli. Since the increase in proteinase activity appears to be regulated by the same variables that control heterocyst differentiation, the activity should provide a useful biochemical marker for the early events of differentiation.
