Nitrogen or sulfur starvation differentially affects phycobilisome degradation and expression of the nblA gene in Synechocystis strain PCC 6803

Nitrogen (N) limitation in cyanobacteria is well documented: a reduced grow th rate is observed, accompanied by a cessation of phycobiliprotein synthes is and an ordered degradation of phycobilisomes (PBS), This leads to a dram atic bleaching phenomenon known as chlorosis, In Synechococcus strain PCC 7 942, bleaching due to PBS degradation is also observed under sulfur (S) or phosphorus (P) limitation, and all three are under the control of the nblA gene product, a 59-amino-acid polypeptide which is overexpressed under N, S , and P starvation (J, L, Collier, and A. R, Grossman, EMBO J. 13:1039-1017 , 1994), Cyanobase sequence data for Synechocystis strain PCC 6803 indicate the presence of two tandem open reading frames (s110152 and s110453) homol ogous to nblA. We cloned the two genes, identified a unique 5' mRNA end sug gestive of a single transcription start site, and studied nblA expression u nder conditions of N or S starvation by Northern hybridization: transcripts were detected only under N starvation (no signal is detected in replete me dium or with S starvation), whether nblA1 or nbLA2 was used as a probe. Mut ations in nblA1 and nbLA2 were constructed by insertion of a kanamycin cass ette; both mutations were nonbleaching under N starvation, Synechocystis st rain PCC 6803 does not bleach under S starvation, consistent with the absen ce of nblA induction in these conditions. These results were confirmed by a nalysis of the PBS components: sequential degradation of phycocyanin and as sociated linkers was observed only under conditions of N starvation, This i ndicates differences between Synechocystis strain PCC 6803 and Synechococcu s strain PCC 7942 in their regulatory and signaling pathways leading to N- and S-starved phenotypes.