Molecular Biology

An Internal Iron Sensor

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Science  05 May 2006:
Vol. 312, Issue 5774, pp. 659-661
DOI: 10.1126/science.312.5774.659d

To a cyanobacterium, photosynthesis is a very important activity. One of the components of the photosystem I (PSI) complex is iron, which can be a limiting nutrient in a marine environment. Consequently, when iron levels are low, Synechocystis makes the iron stress-induced protein A (IsiA) and deploys it in circumferential formation around the valuable PSI centers, where it serves both to enhance the absorption of light and to dissipate excess photoenergy. Dühring et al. describe how, in times of plenty, the expression of isiA is controlled by a 177-nucleotide (nt) RNA, which is called IsrR and is transcribed from the complementary or noncoding strand of isiA. They propose that when iron is abundant, the IsrR RNA binds to the isiA mRNA and targets it for degradation. Because artificially decreasing IsrR levels does not produce an increase in isiA mRNA if enough iron is available, the authors suggest that another control element, such as an attenuator, must exist. In Salmonella, expression of the magnesium transporter MgtA is turned on when internal Mg2+ levels drop, and a critical regulatory element resides in the 5′ untranslated region (UTR) of the mgtA gene. Cromie et al. show that the 200-nt 5′ UTR of mgtA consists of a riboswitch (see also Thore et al., Science Express, 4 May 2006) that adopts a transcription-attenuating, two stem-loop structure when Mg2+ is bound. The 160-nt 5′ UTR region of isiA may harbor an ion- (or iron-) sensor. — GJC

Proc. Natl. Acad. Sci. U.S.A. 103, 7054 (2006); Cell 125, 71 (2006).

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