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Iron stress repressed RNA (IsrR)
Iron stress repressed RNA (IsrR) is a cis encoded antisense RNA which regulates the expression of the photosynthetic protein isiA. IsiA is expressed under iron stress conditions where its expression if activated by Ferric uptake regulator protein (Fur). IsiA enhances photosynthesis by forming a ring around photosystemI which acts as an additional antenna complex.

IsrR is abundant when there is a sufficient iron concentration. IsrR is encoded for within the non-coding stand of isiA gene. Under sufficient iron conditions IsrR binds to its complimentary region which corresponds to the the central third of the isiA mRNA. The resulting duplex RNA is then targeted for degradation. This allows the antisense RNA to act as a reversible switch that responds to changes in environmental conditions to modulate the expression of the isiA protein.

LsrR was originally identified within cyanobacteria but has now been shown to be conserved thougout a number of different species. At present IsrR is the only non coding RNA identified that has a regulatory role on photosynthetic proteins.

IsiA has an inverse relationship with the iron stress repressed non coding RNA (IsrR). IsrR is an antisense RNA that acts as a reversible switch that responds to changes in environmental conditions to modulate the expression of IsiA.