Characterization of the putative iron sulfur protein IdiC (ORF5) in Synechococcus elongatus PCC 7942

Abstract

The IdiC protein (iron deficiency induced protein C) is encoded by orf5 (now called idiC), which is part of the iron-responsive idiB operon of Synechococcus elongatus PCC 7942. The 20.5 kDa IdiC protein has a putative transmembrane helix and belongs to the thioredoxin (TRX)-like [2Fe-2S] ferredoxin family. IdiC has the highest similarity to the peripheral subunit NuoE of the Escherichia coli NDH-1 complex. IdiC expression increased under iron starvation and also in the late growth phase, representing growth conditions, which favor photosynthetic cyclic and respiratory electron transport over photosynthetic linear electron transport from water to NADP+. Attempts to insertionally inactivate the idiC gene generated merodiploid mutants with a strongly reduced IdiC content (mutant MuD) but no IdiC-free mutant. Thus, IdiC seems to be an essential protein for the viability of S. elongatus under the used experimental conditions. Comparative analyses of S. elongatus wild type (WT) and mutant MuD showed that under iron limitation in WT and MuD the amount of the reaction center proteins PsbA and PsaA/B was highly reduced. MuD had a lower growth rate, chlorophyll content, and photosynthetic O2 evolving activity with bicarbonate as electron acceptor than WT. Immunoblot analyses also showed that in MuD, when grown under iron limitation, the amount of the proteins IdiC and IdiB was greatly reduced as compared to WT. As a consequence of the reduction of the transcription factor IdiB, IdiA and IrpA expression were also decreased. In addition, the IsiA protein concentration was lower in MuD than in WT, although the isiA mRNA was equally high in MuD and WT. Another significant difference was the lower expression of the ferredoxin:NADP+ oxidoreductase in mutant MuD under iron limitation compared to WT. A possible function of the protein IdiC in cyclic electron transport around photosystem I and/or in respiratory electron transport will be discussed.

Transcript Profiling Reveals New Insights into the Acclimation of the Mesophilic Fresh-Water CyanobacteriumSynechococcus elongatusPCC 7942 to Iron Starvation

“…Because attempts to insertionally inactivate idiC merely generated merodiploid mutants with a strongly reduced IdiC content but no IdiC-free mutant strain, IdiC is considered to be an essential protein for the viability of S. elongatus PCC 7942. The results of a comparative analysis of S. elongatus PCC 7942 wild type and the idiC-merodiploid mutant MuD suggest a function of IdiC in photosynthetic cyclic electron transport around PSI and/or in respiratory electron transport (Pietsch et al, 2007).…”

“…IdiB is located adjacent to idiA and is part of an operon consisting of orf6, idiC, and idiB that is transcribed in the opposite direction to idiA. Transcription of the idiB operon itself is facilitated by a yet unknown Fe-dependent transcriptional control mechanism (Yousef et al, 2003;Pietsch et al, 2007).…”

“…The gene idiC is part of the Fe-responsive idiCB operon of S. elongatus PCC 7942 and encodes a 20.5-kD protein (Pietsch et al, 2007). IdiC belongs to the superfamily of thioredoxinlike (TRX-like) [2Fe-2sulfur (2S)] ferredoxins and has similarity to the peripheral subunit NuoE of the Escherichia coli NAD(P)H dehydrogenase (NDH)-1 complex.…”

“…IdiC belongs to the superfamily of thioredoxinlike (TRX-like) [2Fe-2sulfur (2S)] ferredoxins and has similarity to the peripheral subunit NuoE of the Escherichia coli NAD(P)H dehydrogenase (NDH)-1 complex. We have shown that IdiC expression increases under Fe starvation as well as during the late growth phase (Pietsch et al, 2007). Because attempts to insertionally inactivate idiC merely generated merodiploid mutants with a strongly reduced IdiC content but no IdiC-free mutant strain, IdiC is considered to be an essential protein for the viability of S. elongatus PCC 7942.…”

“…Moreover, we investigated an IdiB-free S. elongatus PCC 7942 mutant K10 to unravel novel members of an IdiB regulon and an idiC-merodiploid S. elongatus PCC 7942 mutant MuD with a very low content of IdiC as well as of IdiB. The latter strain was included in the investigation because both genes, idiB and idiC, are located next to each other in an operon (Pietsch et al, 2007) and because it was possible to successfully insertionally inactivate the gene encoding the transcription factor IdiB, whereas the idiC-insertionally inactivated mutant never showed full segregation. Because of the suggested essential function of IdiC for the viability of S. elongatus PCC 7942, we included mutant MuD in the investigations to uncover the effects of a highly reduced amount of IdiC in addition to a low content of IdiB on the overall transcriptome during acclimation to Fe-deplete conditions.…”