J. Sauer et al., Nitrogen starvation in Synechococcus PCC 7942: involvement of glutamine synthetase and NtcA in phycobiliprotein degradation and survival, ARCH MICROB, 172(4), 1999, pp. 247-255
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.