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 DpsA-free 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 2'2'-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 wild-type idiA, idiB, and irpA 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.