Unusual Regulatory Elements for Iron Deficiency Induction of the idiA Gene of Synechococcus elongatus PCC 7942

Abstract

Expression of a thylakoid membrane-associated protein called IdiA (iron-deficiency-induced protein A) is highly elevated and tightly regulated by iron limitation in Synechococcus elongatus PCC 6301 and PCC 7942. Although this protein is not essential for photosystem II (PSII) activity, it plays an important role in protecting the acceptor side of PSII against oxidative damage, especially under iron-limiting growth conditions, by an unknown mechanism. We defined the iron-responsive idiA promoter by using insertional inactivation mutagenesis and reporter gene assays. A 67-bp DNA region was sufficient for full iron deficiency-inducible idiA promoter activity. Within this fragment is a palindromic sequence 4 bp upstream of a putative ؊35 promoter element, which resembles the binding site of FNR/CAP-type helix-turn-helix transcription factors. The absence of this palindromic sequence or a 3-bp mutation in a putative ؊10 region eliminated promoter activity completely. A previously identified candidate for a positively acting transcription factor is the IdiB protein, whose gene lies immediately downstream of idiA. IdiB shows strong similarity to helix-turn-helix transcription factors of the FNR/CAP family. A His 6x -tagged IdiB that was overexpressed in Escherichia coli bound to a 59-bp fragment of the idiA regulatory region that included the palindrome. Although the idiA promoter lacks a consensus binding site for the iron-sensing regulator Fur, we attempted to inactivate fur in order to investigate the potential role of this factor. The resulting merodiploid mutants showed constitutive partial derepression of IdiA expression under iron-sufficient growth conditions. We concluded that IdiB is a specific iron-responsive regulator of idiA and that Fur has an indirect role in influencing idiA expression.

Transcriptome response of high- and low-light-adapted Prochlorococcus strains to changing iron availability

“…A fourth MED4-MIT9313 shared gene (also a Prochlorococcus core gene) differentially expressed in both strains is the iron-deficiency-induced gene (idiA), homologous to afuA in Synechococcus PCC7942 and idiA in Synechococcus PCC6301 (Michel et al, 1999(Michel et al, , 2001Webb et al, 2001), was upregulated during iron stress, as observed previously (Katoh et al, 2001;Webb et al, 2001;Singh et al, 2003) and downregulated following rescue. IdiA is hypothesized to be a periplasmic iron-binding protein component of an iron ABCtransporter system including an ATP-binding protein (futC) and permease (futB) (Webb et al, 2001), which were not differentially expressed in either Prochlorococcus strain, consistent with their constitutive expression in Synechocystis PCC6083 (Katoh et al, 2001).…”

Signal Transduction Protein P II Is Required for NtcA-Regulated Gene Expression during Nitrogen Deprivation in the Cyanobacterium Synechococcus elongatus Strain PCC 7942

“…This would explain why those strains containing NtcA showed a very tight repression of the FAM2 reporter, well below the background level of the promoterless control. In the absence of NtcA, there could be unspecific transcriptional activation by NtcA-like transcription factors, such as CysR (26) and IdiB, whose recognition sequences closely resemble the NtcA binding site (24). Therefore, the binding of NtcA to its cognate sites in ammonium-grown cells might prevent cross talk by other transcription factors and might contribute to the repression of transcription from upstream promoters.…”

“…Manipulation and analysis of DNA were carried out according to standard protocols (30). The constructs used to create glnB::luxAB reporter strains were derived from the neutral site II targeting vector pAM1580 (2,24). Plasmid pMP1A, which is identical to pMP1B (8) except that the 1.5-kb PstI fragment carrying the S. elongatus glnB gene is oriented in the opposite direction, was restricted with XhoI.…”

Iron limitation and the role of siderophores in marine Synechococcus

“…PCC6803 and Synechococcus sp. PCC6301 (Michel et al, 2001;Tolle et al, 2002). The capacity for Fe 2+ acquisition encoded by feoB is distributed more sporadically than Fe 3+ genes, being found only in coastal Synechococcus spp.…”

“…PCC6301 and Synechocystis PCC6803 have paralogous copies of IdiA that go the thylakoid or the periplasm (Michel et al, 2001;Tolle et al, 2002) found in Synechocystis sp. 6803 and three coastal marine Synechococcus spp.…”