To learn more about the adaptive response of Synechococcus elongatus PCC 7942 to iron starvation and the role of
DpsA, presumably a protein protecting chromosomal DNA against oxidative damage, we performed a comparative
analysis of S. elongatus PCC 7942 wild-type and a DpsA-free mutant, called K11. Relative to wild-type, the DpsAfree
mutant had significantly higher amounts of phycocyanin and allophycocyanin, even upon iron limitation.
While the Photosystem I activity in mutant K11 remained high under iron deficiency, the Photosystem II activity
dropped severely with respect to wild-type. The DpsA content in wild-type was already fairly high under regular
growth conditions and did not significantly increase under iron deficiency nor in the presence of 0.3 mM 2
dipyridyl
in iron-sufficient BG11 medium. Nevertheless, the absence of DpsA in K11 resulted in a significantly
altered transcriptional/translational activity of genes known to be involved in adaptation to iron starvation. The
amount of isiA/B transcript was about two-fold lower than in wild-type, resulting in a lower 77 K chlorophyll
a fluorescence at 685 nm, implying a lower concentration of Photosystem I-IsiA supercomplexes. While in wildtype
idiA, idiB,andirpA transcripts were highly up-regulated, hardly any were detectable in mutant K11 under iron
limitation. The concentration of mapA transcript, however, was greatly increased in K11 compared to wild-type.
Measurements of acridine yellow fluorescence with intact wild-type and K11 cells revealed that iron deficiency
caused an increased contribution of cyclic electron transport to membrane energisation and ATP synthesis being in
agreement with the formation of the Photosystem I-IsiA supercomplex. In addition, mutant K11 had a much higher
respiratory activity compared to wild-type under iron limitation.


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