SyR22, a new player in the iron-regulatory network of Synechocystis PCC6803

Synechocystis PCC6803 is a unicellular model cyanobacterium that is naturally transformable. The nucleotide sequence of its genome has been established and 3168 potential genes are predicted.

Iron is an important cofactor for many enzymes in cell metabolism, especially in photosynthesis. Therefore, iron is essential for the survival of the cell. Synechocystis responds to iron deficiency by expression of the iron stress-induced protein A (isiA gene) which absorbs light upon chlorophyll binding and forms a giant multimeric ring around PSI to compensate the reduction in the number of PSI under iron limitation. IsiA is regulated by the iron stress-repressed antisense RNA, IsrR.

Recently we found a novel ncRNA, SyR22, which shows the same expression pattern as isiA, that means it is transcribed only under iron limiting conditions. A target prediction made with the program IntaRNA indicated, amongst others, sodB (superoxide dismutase) and isiA as potential tagets of SyR22. We could verify that these genes are indeed targets of SyR22. SodB is normally downregulated under iron deficiency but in a ∆SyR22 mutant the transcript level does not change. Expression of isiA seems to be upregulated by SyR22 at the level of translation, as the ∆SyR22 mutant shows less IsiA protein compared to the wildtype. Therefore, SyR22 acts as activator for isiA expression and in contrast represses the expression of sodB. Our data verify IntaRNA as a very valuable tool in the target prediction of bacterial ncRNAs and provide with isiA the first example in which a trans-acting ncRNA (SyR22) and a cis-antisense RNA (IsrR) jointly control the expression of a bacterial gene.