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

Abstract

The nondiazotrophic cyanobacterium Synechococcus sp. strain PCC 7942 responds to nitrogen deprivation by differentiating into nonpigmented resting cells able to survive prolonged periods of starvation. The degradation of photosynthetic pigments, termed chlorosis, proceeds in an ordered manner in which 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 degradation and cell survival. The P(II) protein, known to signal the nitrogen status of the cells, is most probably not involved in the perception of the nitrogen-starvation-specific signal since in a mutant lacking P(II), chlorosis 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 mRNA and to an increase of nblA mRNA levels, which is characteristic for nitrogen deprivation, suggesting that nitrogen starvation is sensed by a metabolic signal connected to glutamine synthetase activity. However, l-methionine sulfoximine treatment did not induce phycobiliprotein degradation, but led to an immediate cessation of this proteolytic process after its induction by nitrogen deprivation. This suggests that the proteolytic activity elicited by the expression of nblA has to be supported by glutamine synthetase activity.

Acclimation of unicellular cyanobacteria to macronutrient deficiency: emergence of a complex network of cellular responses

“…In the NtcAdeficient mutant, phosphorylation of the P II protein is drastically affected: almost no phosphorylation occurs after nitrogen step-down Sauer et al, 1999), suggesting that P II kinase might be under NtcA control. Furthermore, transcription of the P II -encoding glnB gene is strongly enhanced by activated NtcA under conditions of nitrogen deprivation (see above).…”

“…For example, a modulator of specific responses may also be involved in the control of an apparently general response. Support for such a scenario is provided by the aberrant phycobilisome degradation exhibited by the NtcA mutant, specifically during nitrogen starvation (Sauer et al, 1999); however, when chlorosis is induced by sulfur starvation, NtcA mutants acclimate similarly to the wild-type. Furthermore, induction of the nblA gene following nitrogen stepdown is partially impaired in an NtcA-deficient mutant (Luque et al, 2001).…”

Decoupling of ammonium regulation and ntcA transcription in the diazotrophic marine cyanobacterium Trichodesmium sp. IMS101

“…Its gene product, NtcA, is a 24 kD DNA-binding protein that belongs to the cAMP receptor protein (CRP) family of transcriptional activators (VegaPalas et al, 1992;Flores and Herrero, 1994). Freshwater and marine unicellular, non-N 2 -fixing cyanobacteria show a clear, direct relationship between ntcA expression and low ammonium levels (Frias et al, 1994;Lee et al, 1999;Sauer et al, 1999;Lindell and Post, 2001;Lindell et al, 2005) Marine Synechococcus WH7803 expresses ntcA at basic levels when grown on ammonium. These levels increase to intermediate when exposed to alternative N sources and maximum transcript levels are attained under N starvation (Lindell and Post, 2001;Lindell et al, 2005).…”

A PP2C-type phosphatase dephosphorylates the PII signaling protein in the cyanobacterium Synechocystis PCC 6803

“…Taq polymerase from Roche Diagnostics was used for PCR amplification of DNA. Total RNA extraction and Northern blot analysis were performed as described (28). A 762-bp fragment, containing the whole ORF of pphA, was used as a probe to identify pphA transcripts in 30 g of total RNA loaded on each lane.…”