Nitrogen starvation in Synechococcus PCC 7942: involvement of glutamine synthetase and NtcA in phycobiliprotein degradation and survival

The nondiazotrophic cyanobacterium Synechococcus sp, strain PCC 7942 respon ds to nitrogen deprivation by differentiating into nonpigmented resting cel ls able to survive prolonged periods of starvation. The degradation of phot osynthetic pigments, termed chlorosis, proceeds in an ordered manner in whi ch the light-harvesting phycobiliproteins are degraded prior to chlorophyll . Here, we show that the function of the global transcription activator of nitrogen-regulated genes, NtcA, is required for the sequential pigment degr adation and cell survival. The P-II protein, known to signal the nitrogen s tatus of the cells, is most probably not involved in the perception of the nitrogen-starvation-specific signal since in a mutant lacking P-II chlorosi s proceeded in the same manner as in the wild type. Inhibition of glutamine synthetase with L-methionine sulfoximine led to a rapid decrease of apc mR NA and to an increase of nblA mRNA levels, which is characteristic for nitr ogen deprivation, suggesting that nitrogen starvation is sensed by a metabo lic signal connected to glutamine synthetase activity. However, L-methionin e sulfoximine treatment did not induce phycobiliprotein degradation, but le d to an immediate cessation of this proteolytic process after its induction by nitrogen deprivation. This suggests that the proteolytic activity elici ted by the expression of nblA has to be supported by glutamine synthetase a ctivity.